There is controversy over the health effects of cannabis
Regular cannabis use may be as dangerous as smoking in the long term, claims a UK drug expert.
Professor John Henry, a toxicologist at Imperial College London, says he fears that deaths attributable to cannabis could soar.
There are currently an estimated 3.2 million people in the Britain who smoke cannabis regularly, compared with 13 million tobacco smokers.
Smoking tobacco is believed to cause approximately 120,000 "excess deaths" a year through heart disease, lung cancers and other illnesses.
However, there is no firm evidence of the long-term risks of smoking cannabis.
Studies are clouded by the fact that many cannabis users also smoke tobacco, and it is hard to conduct large-scale studies of individuals who admit using illegal drugs.
The government intends to "downgrade" cannabis from a class "B" to a class "C" drug.
This means that while possession of small quantities of cannabis remains illegal, it is not an "arrestable" offence unless there are aggravating factors, such as use of cannabis near children.
However, Professor Henry, in an editorial for the British Medical Journal, said that if, as many scientists suspected, regular cannabis smoking was as dangerous as tobacco smoking in the long term, the annual death toll from using the drug could be substantial.
He wrote: "It may be argued that the extrapolation from small numbers of individual studies to potential large scale effects amounts to scaremongering.
"For example, one could calculate that if cigarettes cause an annual excess of 120,000 deaths among 13 million smokers, the corresponding figure for deaths among 3.2 million cannabis smokers would be 30,000, assuming equality of effect.
"Even if the number of deaths attributable to cannabis turned out to be a fraction of that figure, smoking cannabis would still be a major public health hazard."
However, there are various reasons why the likely future death toll caused by cannabis smoking is uncertain, not least the lack of large scale studies investigating this.
It is not yet clear whether the high number of younger people who smoke cannabis regularly will continue the habit into middle and later life.
If not, then their risk of premature death would theoretically be greatly reduced, just as a smoker can radically reduce his or her risk of lung cancer by giving up before middle age.
However, some studies suggest that even though cannabis use involves fewer "cigarettes", users tend to draw more heavily on them, increasing the potential damage from each "joint".
Professor Henry said that both cannabis and tobacco released approximately 4,000 chemicals when burned - most of them identical.