Does cannabis lead to taking other drugs?

This is the so-called 'escalation' hypothesis. In the mid-80s research from the US revived interest in this idea; specifically it was claimed that cannabis use tends to lead to heroin use, but the arguments are similar for progression to illicit drugs other than heroin.

Most people who use heroin will have previously used cannabis (though only a small proportion of those who try cannabis go on to use heroin). This could be because cannabis actually does (at least for some people) lead to heroin use, but there are alternative explanations.

For instance, it could be that heroin and cannabis use are both caused by something else in the individual's personality or background that the researchers have not taken into account. Also the studies suggesting cannabis might lead to heroin have been done in Western societies at a time when cannabis is more freely available than heroin. This could mean people tend to use cannabis first simply because they come across it first.

Even if cannabis use did lead to heroin use, there would remain the crucial issue of exactly how this happened. The assumption is that if cannabis leads to heroin, then more cannabis use would result in more heroin use - an argument against legalising cannabis. But the reverse could be the case. For instance, it could be that cannabis use involves people in the buying of illegal drugs, making it more likely that they will meet with an offer of heroin, an offer which some will accept. In this example it would be the illegality of cannabis use rather than cannabis use itself that led most directly to heroin use. The implication is that some heroin use might be prevented by legalising cannabis, even if this meant more widespread cannabis use.

This example illustrates the fact that the mechanism of any link between cannabis and heroin may be as important as whether or not such a link exists in the first place.

All that can be said definitely is that

1. Cannabis use generally precedes the use of other illegal drugs.

2. Cannabis use does not necessarily (or even usually) lead to the use of other illicit drugs.

The unsatisfactory state of affairs with respect to the escalation theory is typical of our knowledge about other ill-effects sometimes said to be caused by cannabis. There is evidence suggesting a number of undesirable consequences of regular use (mental illness, poor motivation, decreased fertility, impaired development in adolescents) but no evidence to either convincingly refute or confirm these suggestions. The illegality of cannabis and the fact that most cannabis users do not approach treatment services makes detailed follow-up studies of cannabis users difficult, and laboratory studies cannot duplicate real life situations. Certainly, widespread cannabis use in the UK and other countries has not been accompanied by correspondingly widespread demand for medical, psychological or social help with cannabis-related problems - but it will never be possible to prove that such problems might not materialise in the future or be found if research techniques improved.