Smoking three cannabis cigarettes a day is as dangerous as having a whole packet of standard cigarettes, say researchers.

Regular smokers of cannabis are at increased risk of developing the potentially fatal lung disease emphysema, claim doctors based in Bristol.

They investigated the smoking habits of four cannabis, or marijuana, users who had advanced emphysema.

Three of the men smoked far less than would usually be associated with the disease, which destroys the surface of the lungs, in standard tobacco smokers.

They said their results questioned the common perception that cannabis was less dangerous than tobacco.

Deeper inhalation

The increased risk could be due to extra volume taken in a draw of a cannabis cigarettes, deeper inhalation of the smoke and extra time the breath is held in for.

The contents of the smoke, other than the mind-altering compound delta-9-tetrahydrocannabinol in cannabis and nicotine in tobacco, are very similar.

But hand-made marijuana cigarettes, made with rolling tobacco, deliver four times more nicotine because of the lack of a filter.

The researchers conducted in-depth case studies of four men aged 27, 35, 44 and 46. One smoked just two cigarettes a day, the second half an ounce of tobacco and the third only smoked cigars.

But the researchers found they were all heavy users of marijuana. The 27-year-old had smoked several pipes a day for some years, the two aged 44 and 46 had two to three marijuana cigarettes a day and the 35-year-old had smoked two a week for 20 years.

Dr Martin Johnson said in the journal Thorax: "Smoking three to four marijuana cigarettes per day produces a comparable histological effect on the airways to smoking 20 tobacco cigarettes daily.

"There is a public perception that marijuana smoking has little adverse effect on physical health, and, given the growing political lobby to legalise marijuana and associated substances in the UK, we hope that our case reports will stimulate further study into their potential lung toxicity."