Are the adverse consequences of cannabis use age-dependent?

Abstract

In recent years attention has focused on a range of effects associated with the recreational use of cannabis that, collectively, may be considered to impact on the quality of life of the user. These encompass broad aspects of psychological and physiological health for cannabis users not only in their current lives, but also prospectively in terms of their future psychosocial development and health. With ongoing debate about cannabis law reform and medicinal uses for cannabis, many countries and organizations have conducted comprehensive reviews in the field (e.g. Hall, Solowij & Lemon, 1994; WHO Programme on Substance Abuse, 1997; Institute of Medicine 1999). There is a consensus that further research is required to inform the various debates and determine the extent or severity of psychological or physiological health risks from ­cannabis. Two research papers in this issue of Addiction offer an interesting counterpoint to the debate. On one hand is a study reporting that cannabis has a significant negative impact on adolescent users who are just commencing their drug-using careers. On the other hand is a study which suggests that by middle adulthood, remitted heavy cannabis users suffer little impact from their past drug-using careers when assessed several decades following cessation. How are we to reconcile these potentially discordant results? First, it must be acknowledged that the samples of cannabis users assessed in each study came from different countries and cultures, had different patterns of initiation to and levels of cannabis use which occurred in quite different eras (1960s−1970s in the United States versus 1990s in New Zealand). They may well have differed in psychological make-up and pre-­morbid psychopathology. These two studies are therefore not directly comparable and do not provide a snapshot of a complementary cohort at different points in time. Nevertheless, these studies provide timely lessons on the differential impacts of cannabis. Fergusson , Horwood & Swain-Campbell (2002) report a further study from their influential longitudinal research on the effects of adolescent cannabis use on psychosocial outcome in young adulthood. They find, from regular assessments of the frequency of cannabis use from ages 14–21 years, that at least weekly use of cannabis at all ages in this period is associated with significant elevations of other illicit drug use, depression, suicidal ideation and attempts and violent or property crime. In a novel approach in this field, the authors applied statistical models to examine the effects of observed and non-observed sources of possible bias on these associations. They argued that even after controlling for known, suspected or measured confounds, associations between cannabis use and psychosocial problems may still reflect the effect of other as yet unknown variables—a perennial problem in determining cause and effect in all manner of research assessing the consequences of cannabis use. Fergusson and colleagues take this matter seriously and raise the standard of typical studies in the field by cleverly showing how observed and non-observed sources of bias can be taken into account using fixed effects regression models that can expand to include observed time dynamic variables relevant to their longitudinal data. These techniques are used to investigate the extent to which changing patterns of cannabis use during adolescence and young adulthood are related to variations in psychosocial adjustment. These confirmed that after controlling for confounds, significant associations held between frequent cannabis use and other illicit drug use, depression, suicidal behaviours and crime. Most striking in these findings were that younger (14–15 years old) users were more affected by regular cannabis use than older (20–21 years old) users. The relative risk of all adverse outcomes except depression was greater for younger regular cannabis users than older users. The association between cannabis use and depression was significant for all ages but did not vary with age. The authors conclude that the adverse effects of cannabis use on ­psychosocial adjustment are most pronounced among younger users and decline with increasing age, thereby drawing our attention to a crucial developmental stage for implementation of interventions, education and prevention strategies. Should we be concerned about the long-range impact of Fergusson et al.'s findings? Using a very different approach, yet one that also controlled for some important fixed-factor sources of bias, Eisen et al. (2002) report no residual adverse effects of past cannabis use on self-reported health measures and quality of life in middle adulthood (age range 38–51, mean 46.2). The strength of this study is that genetic and childhood influences were carefully controlled by using a co-twin control study design. Fifty-six monozygotic male twin pairs were studied; one member of each twin pair was a former frequent cannabis user (reporting an average 1085 days cannabis use over the life-time) a mean 20 years ago, the other a non-user. Neither had any significant use of other illicit drugs. No significant differences were found between the former cannabis users and their non-user siblings on current socio-demographic characteristics, current nicotine or alcohol use, life-time nicotine or alcohol abuse/dependence, past 5-year out-patient or emergency room visits, hospitalizations or medication use for medical problems, past 5-year mental health out-patient use or hospitalizations or health-related quality of life. These findings suggest that remitted, ‘heavy’ cannabis use does not cause major, persistent residual adverse socio-demographic, physical or mental health effects in men with no significant use of other substances. Yet we need to be cautious before accepting this conclusion. The authors acknowledge some of the limitations of their study, including possible participation and recall biases, a relatively small sample size and, importantly, the absence of a physical health examination or pathology tests which may have detected hidden differences between groups. For example, Eisen and colleagues cite the evidence from Tashkin's laboratory of an increased risk of head and neck cancers and abnormalities in bronchial tissue in heavy cannabis users with a minimum 1800 uses. Precancerous changes were not tested for in Eisen et al.'s study, but it is possible that there may have not been any because their sample reported a lower level of overall cannabis use and this may be the primary reason that no adverse outcomes were detected. The majority of the cannabis users in Eisen et al.'s study (83%) reported using regularly for more than 6 years (mean duration 6.1, range 1–22 years); however, 34% had used on 300 days or less during their life-time (73% had used no more than 1000 days), and only nine of the 56 cannabis users (16%) met life-time criteria for abuse or dependence on cannabis. Dependent, heavy (near daily), long-term cannabis users are at greatest risk of developing adverse health and ­psychological consequences (Hall & Solowij 1998) and adverse effects may not become readily apparent or problematic for the user for some time. A number of studies have confirmed (e.g. Stephens et al. 2002) that the mean age of treatment-seeking for dependent cannabis users is in their mid-30s—long after the majority of Eisen et al.'s cohort had ceased using cannabis. Their cohort (born between 1939 and 1955) also started regular cannabis use at a later age (21 years) than has been reported typically in more recent decades (e.g. Degenhardt, Lynskey & Hall 2000). The more typical age of initiation to cannabis use is about 15, precisely the age at which Fergusson et al.'s study showed the adverse effects of cannabis use on adjustment to be most pronounced. This is 6 years earlier than when most of Eisen et al.'s users had started regular use. The cumulative evidence suggests that earlier initiation of cannabis use increases the chance of becoming a daily or nearly daily user of cannabis (Kandel et al. 1986; Fergusson & Horwood 1997) and this, in turn, increases the risk of becoming dependent on cannabis and experiencing problems as a result of cannabis use (Fergusson & Horwood 1997; Hall, Solowij & Lemon 1994). About 10% of those who ever use cannabis and one-third to one-half of those who use daily will become dependent on cannabis and use it despite experiencing problems associated with their use (Hall, Solowij & Lemon 1994; Bachman et al. 1997; Hall & Solowij 1997). There is also evidence that younger cannabis users are now using more potent forms of cannabis at an earlier age, thus increasing the amount of THC consumed, even though the actual concentration of THC in cannabis products has increased only marginally (Hall & Swift 2000). The prevalence of life-time, past month and weekly use of cannabis at different age ranges in the United States are approximately as shown in Table 1 and data from New Zealand, as indeed most other developed regions of the world, are broadly consistent with these patterns. In general, rates of cannabis use, and particularly weekly use, are highest in young adults and then decline steadily with age. Bachman et al. (1997) and Chen & Kandel (1995) have each examined natural patterns of cannabis use from adolescence into adulthood in the United States and both found heaviest rates of use in the early 20s, followed by a steady decline in cannabis use into the 30s, explained by major role transitions. The 1990s saw an increase in cannabis use among youth and there has been a steady decline in the age of initiation among younger cannabis users. For example, in Australia, 21% of cannabis users born between 1940 and 1949 had initiated cannabis use by age 18, compared to 43% of those born in 1950–59, 66% of those born 1960–69 and 78% of those born in 1970–79 (Degenhardt, Lynskey & Hall 2000). For this reason, it is imperative that research continues to investigate the adverse health outcomes associated with dependent, heavy or prolonged cannabis use and we must be continually mindful of the potential public health consequences of the increasing use of cannabis (Hall & Babor 2000). Nevertheless, the results of Eisen et al.'s study accord with the cumulative evidence from other studies, suggesting that the use of cannabis in moderation for a limited number of years is unlikely to have ­serious health consequences for the majority of social ­recreational users. Approximately 90% of experimental or social recreational users of cannabis do not go on to use the drug daily or for a prolonged period; most have discontinued their use by the time they are in their late 20s (Kandel & Davies 1992; Bachman et al. 1997). For the great majority of users, therefore, the likelihood of experiencing long-term adverse effects on physical health is relatively low, and Eisen et al.'s findings support this conclusion. The major concerns for this group, whose use peaks in the early 20s and then declines, would be: an increased risk of motor vehicle accidents while under the influence of cannabis (alone or in combination with alcohol or other substances); the risk of developing schizophrenia for those predisposed; and the exacerbation of symptoms in those with pre-existing diseases (e.g. cardiovascular and respiratory diseases). The risk of immediate adverse mental health consequences may, however, be dependent on the frequency of cannabis use. Heavy use of cannabis has been shown to be associated with an increased risk of depression, anxiety and other psychological markers of distress in both adolescents (Milich et al. 2000) and adults (Troisi et al. 1998; Degenhardt, Hall & Lynskey 2001). Rather than debating whether cannabis does or does not have consequences for mental or physical health per se, perhaps we might think of different parameters of cannabis use as having differential effects and posing ­differential risks during three stages of development. ­Fergusson et al.'s paper forces us to focus sharply on the immediate effects that the use of cannabis has on the young, who are most vulnerable to influences on mental health and psychosocial development. During this period, early initiation of cannabis use and progression to regular use (at least weekly) should ring alarm bells because of the undeniable risks to psychosocial adjustment and development into young adulthood, and to educational achievement. Prevention and education would be the key strategies for intervention at this time. Beyond this critical period of development, the recreational use of cannabis in moderation through a person's 20s may pose lesser risks to health and development but the provision of realistic information to users must emphasize the potential risks associated with acute intoxication and increasing frequency of use. Heavy use (near daily) that shows no signs of diminishing by the late 20s is, of course, a marker for long-term dependent use that is often problematic and may lead to treatment-seeking by the time the person is in their 30s. By now, the use of cannabis will have persisted in spite of the development of multiple forms of social, psychological and physical impairment (e.g. impairment of memory, concentration, motivation, self-esteem, interpersonal relationships, health and employment) related to their cannabis use (Stephens et al. 2002). The development of appropriate early and brief interventions at the community level is crucial, as is the formulation of specific treatments for those who are entrenched in cannabis use and seeking assistance to stop using and manage the multiple associated problems in their lives. By this time, if not before, the cumulative effects of many years of cannabis use emerge to impact upon both mental and physical health. Cognitive impairments and respiratory diseases and cancer are the most probable risks at this point in time. Research into the long-term cognitive effects of cannabis use has shown differential effects to be associated with frequency and duration of cannabis use (Solowij 1998; Solowij et al. 2002). Effects associated with heavy frequency cannabis use (such as a slowing of information processing and chronic bronchitis) must be considered risks by users in their 20s. It is likely that with further research we will achieve a clearer understanding of the delineation of adverse ­consequences associated with either heavy frequency or long-term use of cannabis, their interaction with each other, and with the physiological and psychological processes involved in aging. This will enable a more thorough understanding of the resultant impact of cannabis use on quality of life at different ages.