A single cannabis joint damages the lungs as much as smoking up to five cigarettes at once, researchers say. The drug forces the lungs to work harder by obstructing the air flow and causes chest tightness and wheezing, a study found.

The news comes after research published last week showed that cannabis could more than double the risk of developing psychotic illnesses such as schizophrenia. Experts found that any use of cannabis - even taking the drug just once - was associated with a 41 per cent greater risk. People who smoke the most cannabis were found to be the most likely to suffer a psychotic breakdown marked by delusions, hallucinations or disordered thoughts.

Campaigners renewed their calls for the Government to reverse its decision to downgrade cannabis to a class C drug.

The experts, including Professor Richard Beasley from the Medical Research Institute of New Zealand, published their research today. It will be carried later in the journal Thorax.

A total of 339 volunteers underwent high definition X-rays of their lungs and took part in breathing tests to assess their lung function. They were also asked about their smoking habits. The results showed that cannabis smokers experienced wheezing, coughing, chest tightness and phlegm.

The experts noted that the drug diminished the numbers of small airways that transported oxygen and waste products to and from blood vessels.

The extent of this damage was directly related to the number of joints smoked, with higher consumption linked to greater incapacity, the researchers said.

The effect on the lungs of each joint was equivalent to smoking between two-and-a-half and five cigarettes in one go, they added.