The impact of cannabis on the lungs depends on how it is smoked

One study found a higher risk of lung cancer for those who smoked one joint a day compared with those who smoked 20 cigarettes a day over the same period.

Another found bullous disease - a form of emphysema - occurs 20 years earlier in cannabis smokers.

The studies appear in Respirology and the European Respiratory Journal.

Both studies come at a time when the government is considering whether to change the laws on the possession of cannabis.

Deep inhalation

The lung cancer study was conducted on 79 patients in New Zealand. The risk of the disease rose 8% for each year of smoking one joint a day, and 7% for each year of smoking a packet of cigarettes a day.

The situation is complicated in the UK as the two drugs are almost always smoked mixed together

Steve Rolles

Transform Drug Policy Foundation

Researchers at the Medical Research Institute of New Zealand admitted it was a small study, but said nonetheless "it shows clearly that long-term cannabis smoking increases lung cancer risk".

They speculated the heightened risk may be connected to the fact that the cannabis smoker inhales more deeply and for longer, increasing the amount of carbon monoxide.

This was also thought to be the issue in a second, smaller study of 10 patients who were treated for chronic respiratory problems at the Alfred Hospital in Melbourne.

All admitted intense cannabis use for at least a year. They had developed bullous lung disease, a condition where air becomes trapped in the lungs, at an average age of 41, compared to 65 for tobacco smoking patients.

The campaign group Transform Drug Policy Foundation said it was very difficult to separate cannabis from tobacco use as the two were so often smoked together.

"Variations on the 'cannabis is more dangerous than tobacco' have been emerging sporadically for several decades. Different research using different methodologies has suggested very different conclusions, making such comparisons problematic," said spokesman Steve Rolles.

He added that inhaling smoke from any burning object was intrinsically harmful, and that policy should focus more on educating people about the risks of taking the drug - which can be taken using other methods - in this way.

The British Lung Foundation said the debate about cannabis's effect had tended to focus on the mental impact, overlooking the damage to the lungs.

"Many people don't know that smoking a joint is more harmful to the lungs than smoking a cigarette, as marijuana is often inhaled more deeply and held for longer," said Dr Noemi Eiser, honorary medical director of the British Lung Foundation.

"The New Zealand study highlights the carcinogenic properties of cannabis smoke and it is a great worry that these exist in similar or even greater proportions to tobacco smoke."

The UK Government may decide later this year whether it will reclassify cannabis as a class B drug, having downgraded it to a class C substance in 2004.

There is evidence to suggest that usage has fallen since reclassification occurred.