Smoking a single marijuana joint does as much damage to the lungs as several cigarettes, a study suggests. The drug reduces airflow into the lungs, damages the functioning of the large airways, diminishes the number of small airways and forces the lungs to work harder, according to a report in the London Times.

The effects of each joint were equivalent to between 2½ and 5 cigarettes, according to the New Zealand team who carried out the study.

Defenders of cannabis have argued that it is less damaging than tobacco because many fewer joints than cigarettes are smoked.

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The study, led by Richard Beasley from the Medical Research Institute of New Zealand, recruited 339 volunteers, divided into those who smoked cannabis only (75 people); those who smoked cannabis and tobacco (91); cigarette smokers only (92) and nonsmokers (81). The pot smokers had all smoked a minimum of one joint a day for at least five years, and the tobacco smokers had smoked at least 20 a day for one year.

In Thorax online, the team reports that the volunteers were given breathing tests to measure lung function, and X-rays to investigate damage. The results showed that cannabis smokers experienced wheezing, coughing and chest tightness as well as damage to the airways. But cannabis smokers, unlike cigarette smokers, did not suffer from emphysema.