Comparing cannabis with tobacco

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

To expose the lungs to the same amount of tar as an average cigarette smoker (15-20 a day), cannabis users would have to smoke 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 of four to five times a day.

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

According to the editorial, the more potent forms of cannabis available nowadays carry an increased medical risk but tetrahydrocannabinol, the active chemical ingredient in herbal cannabis, is not known to be harmful to the lungs. Users of more potent forms of cannabis inhale less often and less deeply, while obtaining the same amount of the substance. They may thus benefit from a reduced exposure to potentially harmful tar. Cannabis cannot be considered to be completely harmless, but the arithmetic in this editorial simply does not add up.