People who take paracetamol at least once a week are at higher risk of developing asthma, researchers have found. Daily use of the common painkiller could also cause the condition to worsen, according to a study by doctors at Guy's and St Thomas' hospital, London.

In a study of more than 1,500 people, including over 600 asthmatics, the researchers found that those who took paracetamol every week were 80 per cent more likely to have asthma than those who never took it. Those who took a daily dose were more than twice as likely to be sufferers and were more likely to suffer severely.

Writing in the journal Thorax, the authors suggest that paracetamol could decrease blood levels of the antioxidant glutathione, high levels of which are found in the lining of the airways and the nose. Glutathione is thought to protect the lungs from the harmful effects of pollutants and free radicals, which are high in the lungs of asthmatics.

The authors warn that asthmatics should not substitute aspirin or non-steroidal anti-inflammatories such as ibuprofen for paracetamol as they can cause severe reactions. Instead, they should cut their paracetamol intake if possible.

Smoking three or four joints of cannabis a day is equivalent to a pack a day of 20 cigarettes, doctors say. Researchers from the Glasgow Royal Infirmary report in Thorax that a study of four men who developed the crippling lung disease, emphysema, despite smoking only a minimal number of cigarettes, suggested their disease was due to cannabis smoking.