Long-term Heavy Cannabis Use : implications for health education, Niall COGGANS et al., 2004

Long-term Heavy Cannabis Use : implications for health education


Drugs : education, prevention and policy, 2004, Vol. 11, No. 4, 299–313



There is growing evidence that cannabis can have negative effects on health. While the ongoing debate about the nature and duration of these effects recognizes mild cognitive impairment, the evidence for irreversibility of cognitive impairment and causal links with psychiatric illness is not conclusive. There is undoubtedly potential for impairment of respiratory functioning, but that will depend on lifetime load and in most cases is confounded with tobacco smoking. There is a lack of data that addresses the long-term cannabis user’s perspective. How do long-term cannabis users perceive the impact of their cannabis use on their own lives and what are the policy implications of their experience and perceptions of cannabis use? A recent study of long-term cannabis users explored a number of issues that have relevance for policy in relation to health education interventions. Quantitative data gathered from 405 long-term cannabis users provide insights into the impact of different levels of cannabis use over ten or more years on a range of issues: health; dependence; cannabis-related beliefs and attitudes; and
preferred sources of cannabis-related information. Implications and the need for innovative approaches to cannabis-related health education are discussed.


In a report to the United Kingdom Home Secretary, the (UK) Advisory Council on the Misuse of Drugs (ACMD) recommended that cannabis be reclassified from Class B to Class C under the terms of the Misuse of Drugs Act 1971 (ACMD, 2002). This, according to the ACMD, is to reflect the lesser harm associated with cannabis as opposed to, for example, amphetamine. Moreover, the updated UK drug strategy (Home Office, 2002) stated that the reclassification of cannabis would support the openness, honesty and credibility of a planned new communications programme aimed at driving home the risks of Class A drugs. While the ACMD stated that cannabis is harmful, the nature and degree of potential harm associated with cannabis varies considerably depending on the health issue in question.

There is evidence that cannabis may have negative effects on health. There is also a sizeable proportion of the population that uses cannabis regularly, with 16% of 16–24 year olds and 9% of 25–34 year olds respectively reporting use of cannabis in the last month in England and Wales. Nearly 7% of 16–59 year olds reported past-month use (Condon & Smith, 2003). The same report’s best estimates of the numbers of cannabis users are 3,357,000 for last-year use and 2,068,000 for last-month use. Presumably the prevalence figures would be lower for more frequent use and especially heavy daily use, but that would still translate into significant numbers of people.

This poses considerable problems for social policy in relation to cannabis. Considerable emphasis has been given in the past to criminalization as a means of addressing the cannabis issue. Yet illegality has not prevented the numbers of users reaching current levels. While it might be argued that without prohibition the level of cannabis use would be higher, that is a speculative argument. So there are policy issues not only in relation to legal status—and, recent or proposed changes in a number of countries are towards less severe penalties, not legalization—but also in relation to health education. Findings from a recent study of long-term, heavy cannabis use reported here provide a number of insights into these health-education issues from the users’ perspective.

Cannabis and Mental Health

The research literature on the long-term effects of cannabis on health is mixed. Some studies indicate that cannabis use may have negative effects on respiratory functioning and cognitive impairment, and may result in cannabis dependence syndrome (Hall et al., 1994). Other research has found there to be little effect, if any, of cannabis use on non-AIDS mortality in men and total mortality in women (Sidney et al., 1997). The same study concluded that the relationship between cannabis use and AIDS mortality in men was not causal, but confounded by male homosexual behaviour. Recently, Rey and Tennant (2002) concluded that the evidence confirmed a link between cannabis use and schizophrenia and depression, citing evidence that cannabis use—and particularly early-onset cannabis use—poses greater risk of developing schizophreniform disorders (Arseneault et al., 2002; Zammit et al., 2002), concerns also raised by others (Hall and Degenhardt, 2000). A Dutch study (van Os et al., 2002) reported that cannabis use might elevate the risk of psychotic disorders and lead to poorer outcomes for people with psychotic vulnerability. A recent article in the British Medical Journal (Sheldon, 2003) described the converging evidence from a review of five longitudinal studies, which showed that cannabis use is a risk factor for schizophrenia. There is a need to better understand the risk that cannabis use poses in relation to psychosis, not least to disentangle extent and pattern of cannabis use from underlying biological or social factors.

Bovasso (2001) reported cannabis use as a risk factor in the onset of depression. On the other hand, Arseneault et al. (2002) found no elevation of risk for depression from early-onset cannabis use (age 15), although there was some evidence of elevated risk among those who had used cannabis three times or more by eighteen years. Patton et al. (2002) found that more frequent cannabis use in adolescence, especially daily use, predicted depression and anxiety in young adult women (depression defined as being less severe than major depressive disorder, but clinical intervention indicated). Why this was found with women and not men was not explained. Also of significance in this study was the finding that depression and anxiety in adolescence did not predict cannabis use in later years. Despite this evidence, it is still not clear whether cannabis precipitates or causes psychotic disorder or depression in some people. Moreover, given that these symptoms are found in subsets of cannabis users, the question of specific vulnerability remains unresolved and factors other than cannabis appear to be involved. Grant et al. (2003), in a recent meta-analysis of the residual neurocognitive effects of cannabis use, concluded that their study had ‘failed to demonstrate a substantial, systematic, and detrimental effect of cannabis use on neuropsychological performance’ (p. 687). Regular cannabis users performed less well on tests of memory but the magnitude of this effect was very small, indicating that there may be small negative effects on ability to learn and remember new information. Assessment of seven other cognitive abilities showed no effects.

There is an ongoing debate about the nature and duration of cognitive deficits found after prolonged use of cannabis. For example, Solowij et al. (2002) concluded that long-term cannabis use is associated with cognitive impairments in memory and attention, and that these deficits last beyond intoxication, becoming worse with longer regular use. However, Pope (2002) contends that these deficits, found seventeen hours after last cannabis use, are likely to be a consequence of residual confounding. Moreover, Pope et al. (2001) found that cognitive deficits were detectable at least seven days after heavy use, but these effects had virtually disappeared after twenty-eight days’ washout. A recent review of the evidence (Iverson, 2003) concluded that, while chronic cannabis users show mild cognitive impairment, there is little evidence that these impairments are not reversible and little evidence for a causal link with psychiatric illness.

Cannabis and Dependence