Drug Abuse

Keywords: cannabinoids – cannabis – potency – seizures – THC

Setting the context

Perhaps spurred by rises in treatment admissions and increased knowledge about the health-related harms of cannabis, much has been claimed in the past few years about a change in the potency of cannabis.

There are patterns in recent media coverage of cannabis potency. High-potency herbal cannabis is often contrasted with a purported milder substance smoked in the 1960s and 1970s. European languages use evocative words to label high-strength, indoor-grown cannabis —'skunk', 'nederwiet', `summum'. There is a tendency for coverage of high-potency cannabis to share newspaper pages with extreme cases of cannabis-related psychosis, schizophrenia, treatment admissions or violent crime. Occasionally, high-potency herbal cannabis is linked to discussion of genetically modified crops, subverting identification of cannabis as a 'natural' drug.

There are historical precedents to such alarmism about cannabis potency. Higher strength has been attributed in the past to variants in cannabis products, notably Thai sticks in the 1970s. Authors often refer to an infamous response at a murder trial in 1938 in Newark, New Jersey: when the pharmacologist James Munch was asked about what happened when he himself had tried cannabis, he replied 'After two puffs on a marijuana cigarette, I was turned into a bat'. Such quotes reveal the difficulties we face when trying to discuss cannabis potency from an objective perspective.

This chapter, based broadly on the findings of a longer Insights publication produced by the EMCDDA in 2004, is refreshingly scientific and reassuring in tone. It suggests that overall recorded cannabis potency has not increased dramatically in Europe in recent years.

This is not to say that cannabis potency is a non-issue, but rather that the data in this area are incomplete and far from conclusive. This chapter should be read with the caveat that potency data were — and remain — very limited and that some forms of cannabis now grown in Europe show relatively high potency. More research would be welcome, for example, on how exposure to high potency cannabis affects different user populations, particularly young people and vulnerable groups. In terms of long-term trends, very little is known about the strength of the cannabis smoked in the 1960s and 1970s. And what is striking is that there is considerable variation in the potency of cannabis recorded in Europe. While press coverage tends to concentrate on the strongest THC concentrations rather than average potency, what is constant is the wide range in recorded potency, with only moderate variation in average potency for all cannabis consumed.

One complicating factor is that there has been a recent shift in consumption away from imported cannabis resin to indoor-grown herbal cannabis. While few question that high potency herbal cannabis is increasingly available, particularly in northern Europe, there is a need to track the precise nature of this shift in the market from resin to herb. Are people receiving higher THC doses today than before? Are they smoking fewer joints per session? How are they consuming alcohol and tobacco in combination with high-potency cannabis? Are they smoking joints on more, or fewer days each month? How does a resin joint smoked in the early 2000s compare with a herbal joint smoked today? Can we profile typical consumers of high-potency cannabis, and are they more at risk of problems? Is the shift to herb affected by the drop in supply of Moroccan resin to Europe?

Potency is thus far more complex than the basic task of measuring seized samples of cannabis. More research in particular is needed on titration (the potency–dose relationship) and whether high-potency cannabis is necessarily linked to patterns of problematic use (see Beck and Legleye, Volume 2 of this monograph). While a recent study in the Netherlands provided some findings that high-potency cannabis sourced from Dutch coffee shops can lead to a higher THC concentration in the blood, it also suggested that a core risk group exists (young males aged 18-451 smoking cannabis regularly) which will 'get as high as possible in one session' (Mensinga et al., 2006). Such insights help policymakers to make joined-up decisions that go beyond issues of strength alone, addressing risky use patterns and behaviour over time.

Further reading

Evrard, I. (2007), 'Composition du cannabis: taux de THC et produits d'adultération' in Cordes, J-M. et al. (2007), Cannabis: données éssentielles, OFDT, Paris.
King, L., Carpentier, C., Griffiths, P. (2004), An overview of cannabis potency in Europe, Insights No. 6, European Monitoring Centre for Drugs and Drug Addiction, Lisbon.
Mensinga, T. de Vries, I., Kruidenier, M., Hunault, C., van den Hengel-Koot, C., Fijen, M., Leenders, M., Meulenbelt, J. (2006), Dubbel-blind, gerandomiseerd, placebogecontroleerd, 4-weg gekruist onderzoek naar de farmacokinetiek en effecten van cannabis, RIVM rapport 267002001/2006, Bilthoven.
Mura, P., Brunet, B., Dujourdy, L., Paetzold, C., Bertrand, B., Sera, B., Saclier, B., Deveaux, M., Pepin, G., Perrin, M., Lecompte, Y., Dumestre-Toulet, V., Cirimele, V., Kintz, P. (2006), 'Cannabis d'hier et cannabis d'aujourd'hui. Augmentation des teneurs en THC de 1 993 212004 en France', Anna/es de Toxicologie Analytique 18 (1): 3-6.
Niesink, R., Rigter, S., Hoek, J. (2005), THC-concentraties in wiet, nederwiet en hasj in Nederlandse coffeeshops (2004-2005), Trimbos Institute, Utrecht.
UNODC (2007), 'The emergence of 'new cannabis' and the reassessment of health risks', Chapter 2.3 in World drugs report 2006, United Nations Office on Drugs and Crime, Vienna.

Understanding cannabis potency and monitoring cannabis products in Europe

Leslie King

 

Abstract

The Δ9-tetrahydrocannabinol (THC) content (potency) of herbal cannabis and cannabis resin imported into Europe has remained stable for many years at around 2-8%. Yet cannabis produced locally by intensive, indoor cultivation (sinsemilla) typically has twice as much THC. In some Western European countries, where cannabis resin is the most commonly consumed product and herbal cannabis continues to be imported, the weighted average potency is largely unaffected by these modern developments. However, elsewhere not only is herbal cannabis the dominant product, but that market is largely supplied by sinsemilla. Few countries in Europe have THC measurements stretching back more than five years, and the data are somewhat compromised by analytical difficulties, sampling strategies and the varying nature of cannabis and cannabis resin. Also lacking is any evidence to show that users of high-potency cannabis have higher blood THC levels. The widely publicised claims that cannabis is now 10 or more times more potent than it was 10 or 20 years ago are not supported by the evidence from Europe.

Introduction

The potency of cannabis is defined as the concentration (%) of Δ9-tetrahydrocannabinol (THC), the major active principal of the cannabis plant. As a broad guide, cannabis and cannabis resin typically contain 2-8% THC. However, as will be discussed later, certain products may contain appreciably more. Cannabis grown for fibre production (hemp) will normally contain less than 0.3% THC. Although references will sometimes be found in the literature to 'cannabis purity', this term is ambiguous and could refer to whether or not the material has been adulterated. For example, in the publication Global illicit Drug Trends (UNODC, 2003), 'purity levels' of herbal cannabis and cannabis resin are either clustered around ito 10%, where they probably reflect the THC content, or they are much higher, typically above 50%, suggesting some other measure of purify.

The chemical structure of THC is shown in Figure 1(a). It is one of a large number of related substances known as cannabinoids. Other major constituents of cannabis and cannabis resin are cannabinol (CBN; Figure 1(b)) and cannabidiol (CBD; Figure 1(c)).

It has been suggested that CBD can act as an antagonist of THC (Smith, 2005). This would be of some concern if, as THC levels increased, the CBD concentration stayed constant. However, as far as can be determined from the limited published analytical data, there is a positive correlation between the THC and CBD levels (King et al., 2005a). Cannabis resin has higher relative levels of CBD than herbal cannabis, but the pharmacological significance of this is unclear.

A large fraction of the THC may be in the form of Δ9-tetrahydrocannabinolic acid (THCA). When cannabis is smoked, THCA is converted to THC, although other substances are also formed (Hazekamp, personal communication, 2004). The active isomer Δ8-THC is found in much smaller amounts. The highest levels of THC occur in the resinous material produced by glandular trichromes, mostly situated around the flowers of the female plant. Fertilisation and consequent seed production cause a reduction in the level of THC. Much lower amounts are present in the leaves and in male plants, while the stalk and clean seeds contain almost no THC.

Atmospheric exposure of THC causes oxidation to cannabinol (CBN; Figure 1(b)) and other substances. In cannabis resin, Martone and Della Casa (1990) showed that, even when stored in the dark, the half life of THC was often less than one year, and in some cases THC had disappeared almost completely within two years. In a block of resin, this could lead to variations in the THC concentration between the outside and the inside. The rate of THC decomposition in cannabis at room temperature was estimated as 17% per annum by Ross and ElSohly (1997-1998). Since CBN is almost entirely absent from fresh cannabis, these authors suggest that the ratio CBN/THC could serve as a measure of the age of a sample. The relevance of this to questions of potency can be understood when it is realised that some imported products may have been harvested or manufactured many months before consumption or analysis. By contrast, local production will lead to a fresher product containing more THC.

During the past few years, some concern has been expressed that the potency of cannabis could be much greater than it was. It has been suggested that the THC concentration may have increased so much that the illicit drug now bears little resemblance to the cannabis that was used only 30 years ago. A widely publicised example of this is the statement by the so-called 'drug czar' in the USA (Walters, 2002), published in the Washington Post, that 'parents are often unaware that today's marijuana is different from that of a generation ago, with potency levels 10 to 20 times stronger than the marijuana with which they were familiar'. In a similar vein, Henry (2004) commented on the apparent increase in association between cannabis and deaths recorded as accidents and suicides. He is quoted as saying, 'until the early 1990s, there was less than one per cent tetrahydrocannabinol in most cannabis. Now the most potent form, skunk, contains up to 30 per cent'. Most cannabis is smoked, and according to Ashton (House of Lords, 1998), 'a typical "joint" today may contain 60-150 milligrams or more of THC'.

Meanwhile, in some European countries the numbers of those entering specialised drug treatment centres, who are reported as having cannabis-related problems, have been rising (EMCDDA, 2004) and it has been suggested that high-potency cannabis may be a factor in this trend. High dose cannabis may also be a consideration in evaluating the impact of cannabis on the development of mental health problems such as psychosis, depression and schizophrenia (see, for example, Arseneault et al., 2004).

However, the potency question is not new. Nearly 20 years ago, Cohen (1986) noted that 'material ten or more times potent than the product smoked ten years ago is being used, and the intoxicated state is more intense and lasts longer'. But Mikuriya and Aldrich (1988) pointed out that the cultivation of sinsemilla and its superiority to other forms of cannabis was well known to the British government in India in the 19th century. So what is the evidence that the potency of cannabis has increased in recent decades?

Changes in cannabis potency in Europe

The THC content of cannabis products is routinely determined in many European and other countries. Analyses are usually carried out in forensic science laboratories on behalf of law enforcement agencies, in some cases to provide evidence of cultivation/production. Some information on cannabis potency since 1998 can be found in the EMCDDA's national reports (Standard Table 14). However, these data are rather limited, and no clear trends can be detected. In a recent study (King et al., 2004), much more data were collected, although information on potency trends over five years or more was only available from five countries and a number of methodological problems and information gaps existed. The participants in that survey were asked, by means of a questionnaire, to provide annual mean values of THC percentage in cannabis products, together with information on sample sizes, sampling strategies, method of analysis, the relative consumption of different cannabis products and other information. Despite the limitations, a fairly clear pattern emerged from the survey. Firstly, the potencies of resin and herbal cannabis that have been imported into Europe have shown little or no change, at least over the past 10 years or so. This is hardly surprising since these products have been made by traditional methods that have probably remained the same for generations (see GameIla and Jimenez, this monograph). A brief summary of those findings and a discussion of the implications has been provided by King et al. (2005b). Figure 2 shows the potency of cannabis resin over the period 1997-2003 in the original six countries reported by King et al. (2004) together with data subsequently received from France (OFDT, 2005).

The rapid rise in potency in the Netherlands after 1999 can be explained by the local production of cannabis resin. This material, known as nederhasi, is not only uncommon in the Netherlands, but is almost unknown elsewhere. When the data from the Netherlands are excluded from Figure 2, no overall trend is apparent in the overall mean potency. In the United Kingdom, THC measurements date back 30 years, and the annual mean potencies of cannabis resin as shown in Figure 2 are, if anything, slightly lower than those in the period 1975-1989 (Baker et al., 1980, 1981, 1982; Pitts et al., 1990; Gough, 1991). Cannabis (hash) oil is uncommon in Europe, but its THC content has also shown no clear trend over many years (Baker et al., 1982; Gough, 1991; King, 2001).

What has changed throughout Europe and elsewhere is the appearance, from the early 1990s, of herbal cannabis grown from selected seeds by intensive indoor methods. This material, best described as domestically produced 'sinsemilla' (from the Spanish, 'without seeds'), is also known as 'skunk', 'buds', 'tops' or 'nederwiet'. Its hydroponic cultivation, with artificial control of 'daylight' length, propagation of female cuttings and prevention of fertilisation, certainly does produce cannabis with a greater potency; on average, it may be twice as high as imported herbal cannabis. Further information on the production of sinsemilla can be found in the reviews by Szendrei (1997-1998) and Bone and Waldron (1997-1998).

The THC content of herbal cannabis in the United Kingdom is shown in Figure 3. However, it must be recognised that it is not always possible for a forensic scientist to distinguish the two forms of herbal cannabis. To a large extent, the definition of material as sinsemilla must rely on other circumstances, such as the characteristics of the plantation or 'grow room'. This information may not always be provided by law enforcement agencies and hence some confounding of the two forms may occur. This is illustrated in Figure 3 where the rise in the potency of imported herbal cannabis after 1998 could be an artefact. A similar, albeit modest, rise in the potency of herbal cannabis was also found in Germany (see Figure 4) although no distinction was made between traditional (i.e. imported) herbal cannabis and material produced by hydroponic methods. A small rise in the potency of herbal cannabis was reported by the Czech Republic, but no information was available on the sampling strategy or sample sizes. Further evidence that sinsemilla has a higher potency than imported cannabis can be seen in data produced by the Netherlands (Figure 5). Potency data for herbal cannabis in France are shown in Figure 6, and represent the overall annual mean values for both police and customs seizures (OFDT, 2005). No distinction was made between traditional imported herbal cannabis and sinsemilla, but in each year the mean potency of material examined by the police was close to the mean potency of customs cases. Furthermore, for both herbal cannabis and resin in France, there was little difference in the THC content, according to whether the samples had been seized by law enforcement agencies or the samples had been collected from users (Bello et al., 2005). As with the other countries for which trend data were supplied (Austria and Portugal), little evidence was found for an increase in the potency of imported herbal cannabis.

The most recent data from the Netherlands (Pij'man et al., 2005) show that the THC content of cannabis products has increased even further than illustrated in Figure 5. However, these data need to be interpreted with caution since the Netherlands is anomalous for several reasons. Firstly, in all other countries in the EU the available THC data derive from the analysis of law enforcement seizures. In the Netherlands, the material examined has been purchased in coffee shops: establishments that are permitted to sell small amounts of cannabis (see Korf, this monograph). The samples purchased were generally of better quality material and may not have been necessarily  representative of all cannabis products consumed. This may explain the finding that the cannabis resin purchased for analysis also had a much higher THC content than is seen elsewhere in Europe. Secondly, as noted elsewhere in this report, the relative consumption and origins of cannabis products available in the Netherlands is quite different to other countries.

There is little doubt that, on average, sinsemilla has a higher potency than imported herbal cannabis, but it is also clear that the two potency distributions overlap, as shown in Figure 7. Some samples of imported cannabis are, and always have been, of high potency. The increased THC content of herbal cannabis produced by indoor methods is a consequence of a number of influences. These include: genetic factors (selected seed varieties and cultivation of female plants); environmental factors (cultivation technique, 'pruning' during harvesting, prevention of fertilisation and seed formation); and freshness (production sites are close to the consumer and storage degradation of THC is thereby reduced).

More recent data from the UK for 1999 to 2005 (Figure 8) show that the THC content of sinsemilla may have increased further, probably as a result of continual improvements in technique. Figure 8 also shows, for comparison, the frequency distribution of THC in cannabis resin. Whereas the shape of the distribution of THC in sinsemilla is reasonably symmetrical, the distributions of both imported herbal cannabis and cannabis resin are strongly skewed, with the most common values occurring at the lowest end of the scale.

The increases that have occurred with time in the potency of some types of cannabis must be put into the context of the relative consumption of the various products in different countries. Table 1 sets out estimates of the relative proportion of each cannabis product on the domestic market in recent years. These estimates are shown for those countries where data were either available in the published literature, were supplied directly in response to a questionnaire in the EMCDDA study (King et al., 2004) or were derived indirectly from the relative number of samples examined in each case.

Using both potency data and a knowledge of the relative consumption of different products as shown in Table 1, it is possible to derive the weighted mean potency, that is, the effective THC concentration as would be perceived by the average user. Figure 9 shows the effective potency averaged over all cannabis products in several European countries.

Except for the Netherlands, where it is a dominant product, the limited market share experienced by sinsemilla in other countries suggests that, other aspects of behaviour being constant, users have not been exposed to significantly larger amounts of THC. Although not shown graphically here, UK data for the earlier period 1975 to 1989 indicate that the effective potency in the UK has been around 6% for the past 30 years. In Ireland, where resin is also the main product, the effective potency in 2000 was closer to 4%.

If the effective potency of cannabis had shown an appreciable rise over the past 10 to 20 years then it might be assumed that users would need to consume less cannabis on a weight basis. However, the content of reefer cigarettes (also known as joints or spliffs) examined in the UK over the past 20 years has been remarkably constant (Figure 10). Thus, the typical reefer contains 150-200 milligrams of cannabis or cannabis resin, equivalent to around 10 mg of THC (Humphreys and Joyce, 1982; Bal and Griffin, 2001). Similar results were found in Ireland (Buchanan and O'Connell, 1998). The assertion by Ashton (House of Lords, 1998) that 'a typical "joint" today may contain 60-150 mg or more of THC', suggests a potency of over 50%.

Areas for improvement in analysis and interpretation

If a more accurate picture of potency trends is to be obtained then a number of areas require attention. Apart from a purely quantitative need to obtain more data, these improvements include the following.

Nomenclature of cannabis products

A particular need is the use of an agreed scheme for describing and naming imported herbal cannabis and sinsemilla. At present, a wide variety of terms are in use by authors, including 'seeded cannabis', 'skunk', 'tops', 'buds' and 'nederwiet'. Even
the term 'imported', usually implying a source such as the Caribbean, Africa or Asia, may not be ideal since, in some cases, sinsemilla may be imported from elsewhere in Europe. As noted earlier, confusion may still occur if the growing conditions of the plant material are uncertain, since visual examination of isolated plant material is not always conclusive. Yet, the alternative possibilities for classification (THC content, size of seizure, type of cultivation, such as indoor or outdoor or level of sophistication) also seem unsuitable.

Relative consumption of cannabis products

In most countries, estimates of the relative consumption of different cannabis products are based largely on seizure data. Such data have limits and may not directly reflect the relative market share of different cannabis products or availability as experienced by drug users. One way forward would be to complement statistics from drug seizures with data from user surveys carried out at the retail level. This might also include information from seed suppliers and shops selling growing equipment/paraphernalia.

Proxy measures of potency

Few countries have published data on the herbal cannabis or cannabis resin content of reefers. This information would be useful as a proxy measure for potency as well as a means of tracking methods of consumption (i.e. use with or without tobacco). In Europe, information is collected routinely by the EMCDDA on drug prices at retail level. However, the quality and comparability of this information needs to be reviewed and standard methods for collection and reporting developed. Data from the Netherlands suggest a close relationship between potency and price (Trimbos Institute, 2002).

Extent of domestic production

It is important to have a better understanding of the extent of domestic cannabis production, the different types of production methods used, as well as the use of domestically produced cannabis products compared with imported products and how this varies within Europe and over time. Furthermore, home-produced cannabis may not always benefit from hydroponics or other sophisticated growing techniques.

Data presentation

When compiling data, many laboratories calculate simple mean values (often called averages: the sum of all values divided by the number of values). In a few cases, weighted means may be calculated. These take account of the fact that not all samples may be of equal size. Few authors consider whether the distribution of potency is normally distributed or if other measures of central tendency such as the median or mode would be better. Ideally, data collections should always indicate details about the sampling strategy, sample size, the mean, and where possible more detailed descriptive statistical information (e.g. mode and median values, standard deviation and treatment of outliers).

Sampling

Sampling is probably the most important variable relating to the measured potency of cannabis. Cannabis, and to a lesser extent cannabis resin, is an extremely inhomogeneous material. As noted earlier, the THC content of different parts of the plant shows considerable variation. As well as the flowering tops of the female plant, where most of the THC is located, a sample may contain varying amounts of stalk, seeds and leaves, none of which contains much active drug. If potency is to be compared between different laboratories, or even within the same laboratory at different times, then a standard method of sample preparation is required.

Laboratory analysis

Assuming that the THC in cannabis and cannabis resin can be solvent-extracted with total, or at least a known, efficiency, then most laboratories use gas chromatography, often with flame-ionisation detection (Raharjo and Verpoorte, 2004) to determine THC concentration. This has the merit that the naturally occurring precursor (THCA) is decarboxylated to THC, just as occurs during smoking. Cannabinoids can also be determined by high-performance liquid chromatography, a method suited to profiling ('chemical fingerprinting') and the separate measurement of THCA. To measure the total THC content by HPLC, the sample must be heat treated before analysis (Lehmann and Brenneisen, 1995; Rustichelli et al., 1998; Kanter et al., 1979).

The major issue to arise in the analysis of THC concerns the accuracy (closeness to the 'true' value) of the measurement process. Poortman van der Meer and Huizer (1999) claimed that in a series of proficiency tests, using standard solutions of THC, and organised in 1997 for 30-40 European laboratories, the relative standard deviation was about 29%, whereas cocaine and amphetamine gave less than 5% and 8% respectively. This means that around one-third of results for THC were either more than 29% above or more than 29% below the mean value. It is clear that even worse precision could be expected if the measurement error caused by the sampling and extraction process were to be included.

As a reference standard, THC is usually only available from chemical suppliers in the form of an ethanolic solution and may be labelled, for example, as `approximately 95%'. Not only could confusion arise if analysts assume the concentration to be 100%, but Poortman van der Meer and Huizer (1999), using the response of a flame-ionisation detector, found that one sample of a commercial THC solution had only 90% of the concentration of a different commercial solution. These authors recommended that THC quantification should be based on cannabinol or cannabidiol as the internal standard and a correction made for the expected detector response from the effective carbon number of the respective substances. They claimed that this method had been used in Germany for the past 10 years. It was also the method used by Maguire (2001) to study the cannabinoid content of (mostly fibre-type) cannabis in Ireland. However, as far as could be determined in the EMCDDA study (King et al., 2004), other European laboratories continue to prepare standard dilutions of stock THC solution to construct calibration curves.

To a large extent, and excluding the special situation of locally produced Dutch nederhasj, the cannabis resin consumed in Europe in recent years has originated mostly from North Africa, with smaller amounts coming from South-West Asia. Since resin is rarely adulterated, it could be argued that, in any given year, all laboratories have been measuring broadly similar material. Despite the variation of THC content in cannabis products discussed above, if those laboratories had made sufficient measurements, then the mean potency of cannabis resin in any year should be found to be similar for all countries. Inspection of Figure 2 shows that not only is there no time trend, but there is considerable variation in the reported THC levels, both against time in any one country and between countries at any one time. It is not obvious why there should be consistently less THC in cannabis resin in Portugal compared with cannabis resin in, for example, the Czech Republic or France. This finding raises questions about the accuracy of measurement of THC in different laboratories. In other words, if all analysts had used the same THC reference standard for instrumental calibration, then these differences might not have occurred.

Pharmacology

In Europe, cannabis is normally smoked often in a mixture with tobacco in a reefer cigarette, but some is smoked in a water pipe (a bong). By contrast, in the USA where little resin is consumed, cannabis is usually smoked alone. Furthermore, the sources of cannabis and cannabis resin consumed in North America are not the same as those in Europe. Nearly all studies on the smoking of cannabis and its relation to potency have been carried out in North America, and it is clear that this research may not translate well into the European situation. Thus Matthias et al. (1997) found some evidence that those who smoke more potent cannabis are less exposed to noxious smoke components than those who use less potent forms. But in Europe, or at least in Ireland and the United Kingdom, where a reefer cigarette typically contains only 150-200 milligrams of cannabis (Buchanan and O'Connell, 1998; Bal and Griffin, 2001; Humphreys and Joyce, 1982), much of the tar, carbon monoxide and other combustion products will derive from the concomitant tobacco.

The concerns that have been expressed about a possible rise in cannabis potency often assume that users will necessarily consume more THC, but the evidence for this is equivocal. If the potency of cannabis products has shown a marked increase, then it might be expected that the typical user would need to consume less on a weight basis to achieve the desired effect. Given a choice, users preferred cigarettes with a higher THC content (Chait and Burke, 1994; Kelly et al., 1997). Ashton (1998) also argued that users would not titrate the dose of THC from cannabis in contrast to tobacco smokers. However, Heishman et al. (1989) found that those smoking cigarettes with a higher THC content tended to have a lower inhalation rate than control subjects. Yet little research has been conducted, particularly in Europe, to answer a crucial question: do those smoking high potency cannabis have higher blood levels of THC?

However, even if the strength of some forms of cannabis has increased, and even assuming that, as a consequence, users do have higher blood levels of THC, then it cannot be concluded that this will translate into a greater harm to the individual. Experience with alcohol suggests that the health consequences are not simply related to the alcohol concentration of what is consumed, but rather it is the total quantity of alcohol consumed that is important. As Hall et al. (2001) note, age of onset of use and frequency of use are likely to be more influential than changes in potency in determining consumption levels.

Medicinal cannabis

In any discussion about the health impact of high-potency cannabis, mention should also be made of cannabis used for medicinal purposes (see also Witton, this monograph). In the Netherlands, herbal cannabis is available as a prescription medicine (Office of Medicinal Cannabis, 2004). It is indicated for multiple sclerosis, certain types of pain and other neurological conditions. Patients are advised to consume the cannabis by means of a hot water infusion. However, Hazekamp (personal communication, 2004) has found that, even in boiling water, the conversion of THCA to THC can take some hours and other byproducts are formed. Remarkably, one of the forms of this medicinal product, known as 'cannabis flos', has a nominal THC content of 18% and is locally produced by the same intensive indoor methods that are used for illicit cultivation. Not only is high-potency cannabis considered suitable as a medicinal product, but an assessment carried out by the Dutch Coordination Centre for the Assessment and Monitoring of New Drugs concluded that (illicit) higher-potency cannabis products did not pose any additional risk than those present for cannabis products as a whole, either to the individual, to society, to public order or criminality (W. Best, personal communication, 2004).

Conclusion

The potency, that is, the THC concentration, of herbal cannabis produced by intensive indoor cultivation can average over 10%, compared with an average of 5% for both imported cannabis resin and cannabis grown by traditional methods. For all cannabis products there is a wide variation about average values and some users will inevitably have been exposed in almost random fashion to higher than normal THC levels in their careers. The evidence from Europe does not support the widespread claims that cannabis potency is now 10 or more times greater than it was in earlier periods. Although not part of this present review, experience from outside Europe (King et al., 2004) comes to a similar conclusion.

Acknowledgements

Alison Yeo of Forensic Alliance (Culham, UK) kindly provided unpublished THC data from police seizures.

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