Cannabis: unanswered questions

In keeping with most commentators and DrugScope, Mary Riddell in Sunday's Observer (22nd July) criticised the decision to review yet again the classification of cannabis as an unnecessary distraction from the real problems surrounding drug use in the UK. The only rationale for the decision appears to be the continuing belief that cannabis is stronger than it used to be and so hence poses a greater threat to mental health. Mary Riddell acknowledges DrugScope's view that there is no 'robust evidence' for this, but points out nonetheless that she knows of young people who have stopped smoking cannabis because they became frightened of the effects. So what might be happening?

It could be that across the board, cannabis being smoked in the UK, either home grown herbal cannabis or imported resin is much stronger than it used to be. But the only way to establish the general level of THC content is to actually spend some money finding out because the Forensic Science Service does not routinely test for cannabis potency - this is not required for the purposes of criminal prosecution. However if government policy (and public perception of risk) is being driven by the potency issue, then surely some simple testing of samples collected nationwide could be done? Obviously you cannot then match the results with samples from five or ten years ago, but you could then search the forensic literature for any indications of trends. This would not produce definitive answers, but at least it would add to the evidence base and help inform the debate.

What else might be happening? Many of those working in the field are familiar with the idea that how drugs might affect the individual is not simply the product of the drug itself. Effects can be influenced by what the user expects to happen (called 'set') and the situation in which the drug is being used (called 'setting'). The literature suggests that of all the drugs, what happens to somebody under the influence of cannabis is particularly susceptible to the drug-set-setting formula. In effect, users have to 'learn' how to react to cannabis and this explains why many first time users say they experienced no effects at all from smoking cannabis. It is not impossible that some young people report experiencing stronger effects from cannabis not because the drug is actually stronger but because they believe it is stronger through a combination of persistent media assertions that it is and the reinforcing beliefs of their friends and those selling cannabis who would want customers to believe that their product was 'the real deal'.

But even with some more flesh on the bones of these issues, we would still be left with unanswered questions about how cannabis toxicity relates to health problems and it may be beyond the scope of medical ethics to answer these points conclusively. Which ultimately brings us to the position that any sensible policy should consider what is most likely to happen to most people who choose to smoke cannabis based on the best possible evidence available and not be unduly influenced by unsubstantiated assertions of risk.

Harry Shapiro