Comparing cannabis with tobacco: Arithmetic does not add  up

doi:10.1136/bmj.327.7407.165

BMJ 2003;327:165 (19 July), doi:10.1136/bmj.327.7407.165

Letter

Comparing cannabis with tobacco

Arithmetic does not add up

EDITOR—The editorial by Henry et al comparing cannabis with tobacco implies that in the future as many as 30 000 deathsa year in Britain may be caused by smoking cannabis.¹ Butthis conclusion seems to have been based on a series of questionable assumptions.

To expose the lungs to the same amount of tar as an averagecigarette smoker (15-20 a day), cannabis users would have tosmoke four to five times a day, every day of the week. In fact,surveys show that a large majority in Britain are occasional"weekend" users, and few fall into the high use category offour to five times a day.

Another important factor is that unlike cigarette smokers mostcannabis smokers tend to stop when they reach their 30s. Longterm surveys of cigarette smokers showed that those who stopbefore the age of 35 had only a very slightly increased riskof lung cancer. If the risks of cannabis smoking equate tothose of tobacco and most users give up before the age of 35they may run little additional medical risk.

According to the editorial, the more potent forms of cannabisavailable nowadays carry an increased medical risk but tetrahydrocannabinol,the active chemical ingredient in herbal cannabis, is not knownto be harmful to the lungs. Users of more potent forms of cannabisinhale less often and less deeply, while obtaining the sameamount of the substance. They may thus benefit from a reducedexposure to potentially harmful tar. Cannabis cannot be consideredto be completely harmless, but the arithmetic in this editorial simply does not add up.

Les Iversen, professor

Department of Pharmacology, University of Oxford, Oxford OX1 3QT les.iversen{at}pharm.ox.ac.uk

Competing interests: None declared.