Cannabis may be safer than was thought - but only if it remains illegal, a report by a health expert suggests.

Recent estimates that cannabis causes up to 30,000 deaths a year - a quarter of the number caused by smoking tobacco - are likely to be exaggerated, Stephen Sidney, associate director of clinical research at the California health maintenance organisation Kaiser Permanente, said.

Writing in the British Medical Journal, Dr Sidney said that two long-term studies of the drug, involving a total of more than 100,000 people in Sweden and the US, found no increase in deaths. Furthermore, unlike other drugs both legal and illegal, there has been no known lethal overdose from cannabis.

The harmful effects of tobacco, with which cannabis is often compared, are long term. Smoking is known to contribute to heart disease, one of the Western world's biggest killers. Nicotine has a damaging impact on the heart but there is no nicotine in cannabis.

Cannabis was also exonerated as a cause of heart disease by a study that showed no increase in calcium deposits in the coronary arteries of young adult users of the drug - an indicator of thickening of the arteries that can lead to heart attacks.

"Although the use of cann-abis is not harmless, the current knowledge base does not support the assertion that it has any notable adverse public health impact in relation to mortality," Dr Sidney said.

But he said the long-term effects of cannabis were not known because users had not been followed into middle and old age. Most give up the drug in their twenties and thirties and this is likely to minimise harmful effects. But if the drug were legalised it is possible that more people would continue using it for longer. "We cannot assume that smoking cannabis would continue to have the same small impact on mortality ... if its use were to be decriminalised or legalised," Dr Sidney said.