People who smoke a single cannabis joint could be doing as much harm to their lungs as someone who smokes five cigarettes in one go, experts claim.
Researchers found that the drug forces the lungs to work harder than normal as it obstructs air flow, and causes chest tightness and wheezing.
They divided 339 volunteers into four groups: cannabis only smokers; combined cannabis and tobacco smokers; those who only smoked tobacco and nonsmokers.
All the volunteers underwent high definition X-rays of their lungs, and took part in breathing tests to assess their lung function.
When the results were analysed, those who smoked cannabis were found to have less of the small fine airways that transport oxygen and waste products to and from blood vessels than those who do not use the drug.
The experts, including Professor Richard Beasley, from the Medical Research Institute of New Zealand, published their findings ahead of print in the journal Thorax.
They concluded: "The most important finding was that one joint of cannabis was similar to 2.5-5 tobacco cigarettes in terms of causing airflow obstruction."
Dr Keith Prowse, chairman of the British Lung Foundation, said: "This research confirms that cannabis poses a serious health risk to the lungs and smoking a joint can be more harmful to the lungs than smoking a cigarette."