Researchers in America have found that blood from marijauna smokers has nearly three times as many DNA mutations as that of non-smokers.

Marijauna smoke contains many of the carcinogenic chemicals found in cigarette smoke, but, unlike tobacco, has never been linked to the kinds of genetic mutations associated with lung cancer.

But Dr Marinel Ammenheuser and her colleagues at the University of Texas Medical Branch now have evidence that marijauna smoking causes the same kind of damage to DNA as tobacco.

Damaged cells

Dr Ammenheuser said her group studied a group of pregnant women who were marijauna smokers, but who did not use other drugs, and did not smoke tobacco.

She said: "We did a test to detect changes in a particular gene in the DNA in their white blood cells, and found that the marijuana smokers had three times as many DNA damaged cells as the non-smokers.

"We did the same test on the blood cells for the umbilical cords of the new-borns of some of the marijuana smokers. These infants likewise had three times as many damaged cells as those of non-smokers."

Hundreds of chemicals are produced when marijuana leaves and buds are burned in the smoking process.

It is therefore very hard to say which are damaging the DNA. However Dr Ammenheuser is confident that one group of chemicals called poly-aromatic hydrocarbons are responsible.

Worse than cigarettes

Marijuana: worse than cigarettes?

Dr Ammenheuser said: "We can't actually make statements about cause and effect from our study, but we can say that marijuana smoking probably increases your risk of getting things like lung cancer, in much the same way that we know that tobacco smoking increases this risk.

"My research indicates that smoking just a few marijuana joints seems to cause as much DNA damage as ten or more cigarettes.

"I think it is because marijuana smokers hold the smoke in their lungs for longer and do not use low tar, filtered marijuana joints.

"And they generallly smoke the joints down to a very small butt, which increases their exposure to the cancer causing chemicals."