Cannabis may be responsible for the deaths of 30,000 people a year if the ill-effects of smoking the drug are comparable to those of smoking tobacco, researchers say.

With an estimated 3.2 million users of cannabis in the UK doctors have calculated the death toll based on the 120,000 deaths among the 13 million smokers of tobacco.

Professor John Henry of Imperial College, London, and colleagues say in the British Medical Journal: "Even if the number of deaths attributable to cannabis smoking turned out to be a fraction of the 30,000 we believe could be possible, cannabis smoking would still be described as a major health hazard. If we also add in the likely mental health burden to that of medical illnesses and premature death, the potential effects of cannabis cannot be ignored."

The number of cannabis smokers is increasing and regular cannabis smoking is known to be associated with an increased risk of schizophrenia and depression. Although people smoke fewer cannabis cigarettes than tobacco cigarettes, the larger puff volume and deeper inhalation that characterises cannabis smoking means each cigarette is likely to be more damaging.

The authors draw attention to the different ways in which tobacco and cannabis are treated. They write: "Prevention and cessation are the two principal strategies in the battle against tobacco. At present there is no battle against cannabis and no clear public health message."

However, doubt was cast on the warning yesterday by anti-smoking charities who pointed out most cannabis smokers give the drug up in their 30s, but tobacco smoking goes on for decades because of the addictive nicotine.

The Government's Advisory Council on the Misuse of Drugs noted in a report published last year: "In general, cannabis users smoke fewer cigarettes per day than tobacco smokers and most give up in their 30s, so limiting the long-term exposure that we now know is the critical factor in cigarette-induced lung cancer".