Smoking a joint a day is just as bad for you as smoking a pack of cigarettes a day, a study from New Zealand indicates.

Smoking a joint a day for one year boosted the likelihood of developing lung cancer by 8 per cent, Dr Richard Beasley, of the Medical Research Institute of New Zealand in Wellington, and his colleagues found. The risk for cigarette smokers increased by 7 per cent for every year they smoked a pack a day. The heaviest pot smokers were at nearly sixfold greater risk of lung cancer compared with people who didn't smoke marijuana.

"The balance of evidence would suggest a positive association between cannabis and lung cancer," Beasley and his team conclude in the February issue of the European Respiratory Journal.

Evidence for the lung cancer/pot smoking link has been mixed, while legal issues as well as difficulty in quantifying marijuana use make studying pot's health effects difficult, the researchers note. Since New Zealand has high rates of both pot smoking and lung cancer, and the nation's marijuana users rarely mix weed with tobacco, it "represents an ideal country in which to study the association between cannabis and respiratory tract cancer", the researchers say.

The researchers matched 79 lung cancer patients with 324 healthy controls, all of whom were younger than 55. Study participants' smoking habits were quantified using "joint years" (one for every year the participant smoked a joint daily) or "pack years" (one for every year the participant smoked a pack daily).

Overall, marijuana smoking, which the researchers defined as having smoked at least 20 joints in one's lifetime, didn't increase lung cancer risk. However, people who had more than 10.5 joint years under their belts were 5.7 times more likely to be lung cancer patients. Based on the findings, the researchers say, about 5 per cent of lung cancer cases among people 55 and younger in New Zealand could be attributed to pot smoking.

The researchers think worldwide efforts to reduce tobacco smoking "may need to include greater initiatives to reduce cannabis smoking and should be directed particularly at young people".

Reuters