In the 1990s, cannabis is in the news again as research reveals an upturn in use and governments struggle to develop a policy response that weighs the potential harm of the drug against the potential harm of drug policy itself.

Cannabis--sold as marijuana, hashish and hash oil--is the most frequently used illicit drug in Canada. Roughly one in four Canadian adults report having used cannabis at some time in their lives. And use has been on the rise among young people. For example, a 1997 Addiction Research Foundation (ARF) survey found that 25 percent of Ontario junior high and high school students used cannabis in the previous year, up from 13 percent in 1993.

One feature of the renewed interest in cannabis is the frequency with which questions on the subject have been put to political candidates. Their responses--often including admissions of cannabis use--are typically lighthearted, but the humor is perhaps lost on the hundreds of thousands of Canadians with criminal records for cannabis possession.

In October, 1995, Canada's House of Commons passedThe Controlled Drugs and Substances Act, a law criticized for its continuing harsh approach to cannabis possession.

To deal with ongoing concerns about cannabis policy, the basic questions that must be addressed remain the same:

What do we know about the health risks associated with cannabis use?

What is the most effective and least costly way to minimize these risks?

What is the most effective way to minimize potential harms resulting from our drug policy response?

Some health consequences of cannabis are clearly known, while others--such as the effects of chronic exposure--are less obvious..

There is no doubt that heavy cannabis use has negative health consequences. (For detailed documentation of research and reference material, please see Hall et al, 1994, and WHO, in preparation). The most important effects are:

Respiratory damage: Marijuana smoke contains higher concentrations of some of the constituents of tar than tobacco smoke. As well, it is hotter when it contacts the lungs and is typically inhaled more deeply and held in the lungs longer than tobacco smoke.

Research has shown a link between chronic heavy marijuana use and damage to the respiratory system similar to that caused by tobacco..

Long-term marijuana smoking is associated with changes--such as injury to the major bronchi--that leave the lungs open to injury and infection. Frequent, heavy use has been linked with bronchitis (Bloom et al., 1987; Tashkin et al., 1988.) There is no established link between marijuana smoking and lung cancer. But case reports of some cancers in young adults with a history of cannabis use are of concern. (Polen et al., 1993).

These adverse effects are, of course, related to smoking the drug, and don't occur when cannabis is eaten.

Physical co-ordination: Cannabis impairs co-ordination. This brings with it the risk of injury and death through impaired driving or accidents such as falls.

North American studies of blood samples from drivers involved in motor vehicle crashes have consistently found that positive results for THC (the mood-altering ingredient in cannabis) are second only to positive results for alcohol. However, blood levels of THC do not demonstrate that a driver was intoxicated at the time of the accident..

In addition, many drivers with cannabis in their blood are also intoxicated with alcohol.

Experimental studies of driving that show that cannabis use can impair braking time, attention to traffic signals and other driving behaviors . The studies found that subjects appear to realize that they are impaired, and compensate where they can. However, such compensation is not possible when unexpected events occur, or if the task requires continued attention.

Pregnancy and childhood development: Cannabis use by women who are pregnant may affect the fetus. As with tobacco smoking, risks such as low birth weight and premature delivery increase with use.

The longer-term effects on children whose mothers smoked cannabis while pregnant appear to be subtle. Recent research suggests that exposure to cannabis in the womb can affect the mental development of the child in later years. By age four, for example, offspring of women who used cannabis regularly showed reduced verbal ability and memory. By school age, decreased attentiveness and increased impulsiveness were also found in children whose mothers used cannabis heavily (Day et al, 1994; Fried, 1995).

Memory and thinking: The effects of cannabis on memory appear to be variable, and may depend on the test that is used. Overall, the effects seem to be modest. However, it's not yet known whether chronic use would produce serious impairments of memory, particularly is such use occurs during development. Several years ago, studies of adult cannabis users suggested that the drug has little effect on cognitive function. More recent research has demonstrated that long-term use produces deficits in the ability to organize and integrate complex information (Solowij et al., 1995).

Psychiatric effects: Cannabis use has been linked to a number of psychiatric effects. The most significant is called cannabis dependence syndrome. A person with this condition will continue to use the drug despite adverse effects on physical, social and emotional health (Anthony and Helzer, 1991). Impairment of the person's behavioral control, combined with effects on thinking and motivation, can adversely affect a person's work or studies. The risk of dependence increases with use. It has been reported that one-third to one-half of those who use cannabis daily for long periods may become dependent.

There is clearly a link between cannabis use and schizophrenia, but it is not yet known whether cannabis use triggers schizophrenia, or whether schizophrenia may lead to increased cannabis use (Andreasson et al., 1987; Andreasson et al., 1989). Health professionals have identified a condition of "cannabis psychosis" following heavy use of the drug (Chaundry et al., 1991; Thomas, 1993). The condition disappears within days of abstinence..

However, this disorder has not been well defined, and it is not clear that it differs from the effects of high doses of the drug.

Reference has also been made to an "amotivational syndrome" resulting from extensive cannabis use. While heavy use of cannabis may interfere with motivational, the existence of a syndrome with identifiable symptoms outlasting drug use and withdrawal has not been demonstrated. (This question may have been clouded by studies of effects of cannabis use on educational performance in adolescents, in which individuals most likely to use the drug may have lower motivation to succeed academically.)

Hormone, immune and heart function: Research has shown that cannabis can also alter hormone production, and affect both the immune system and heart function. The implications of these findings for human health are unclear at present.

The link between cannabis and the use of other drugs is also of concern. In particular, people have questioned whether cannabis acts as a "gateway drug" to heroin, cocaine or other drug use.

There is a statistical link between the use of cannabis and other drugs. Cannabis users are more likely to use tobacco and alcohol, for example. They are also more likely to try other illicit drugs than those who have never used cannabis. As well, the earlier a person uses cannabis and the more he or she consumes, the greater the likelihood that the person will use other illicit drugs.

The reason for this link is less clear. It's likely, however, that the use of cannabis does not in itself lead to the use of other illicit drugs. For example, roughly one in four Canadians has used cannabis, yet only four percent have ever used crack or cocaine. Similarly, just two percent have ever used amphetamines and about half of one percent have ever used heroin (Health Canada, 1995).

A more likely explanation is the cannabis use may be one of many social and cultural factors--including family relationships, mental health, peer influences, social attitudes and beliefs--associated with a higher likelihood of the use other substances as well. In other words, the same factors that contribute to cannabis use may lead a smaller number of individuals to go on to other illicit drugs. This may also explain the statistical link between cannabis use and lower academic and professional achievement and other personal and social problems.

Cannabis and other street drugs are also linked by the very fact that they are illegal--a dealer who sells cannabis may also offer other drugs.

Many of the negative effects of cannabis are associated with long term heavy use. As mentioned earlier, however, most Canadians who use cannabis do so sporadically and in small amounts. Certainly, the typical pattern of cannabis use is much different from that of cigarette smoking. For most marijuana users, damage to the lungs is therefore likely to be limited.

Given the current patterns use, probably the most important health effects of cannabis use are:

injury or death resulting from intoxication--for example, from a traffic crash

respiratory disorders and ailments linked to heavy use

dependence on cannabis, arising in a small proportion of users.

By any accounting, the impact of health problem linked to cannabis is much less than that resulting from alcohol or tobacco use. Survey data from the US., for example, show that dependence on nicotine among smokers is several times more prevalent than cannabis dependence among marijuana users (Kandel et al., 1997). Moreover, the legal drugs tobacco and alcohol account for the bulk of the economic costs of substance use. For example, a recent Ontario study found that annual health care costs resulting from cannabis use were small ($8 million) when compared to those for tobacco ($1.07 billion ) and alcohol ($442 million). (Xie et al., 1996; Unpublished analysis of economic cost date, ARF, 1997)