Makers of the anti-flu drug Relenza have written to every GP in Britain warning that the spray could trigger breathing problems in some patients.

Glaxo Wellcome said its drug could have possible side effects for those with asthma and lung disease after a handful of US patients were affected.

The company said the health alert was a precaution, and that less than 10 people had suffered side effects out of 500,000 who have used the drug.

Relenza, the first drug of its kind to be given a licence in the UK, was involved in controversy last year when a government advisory body told doctors not to prescribe it on the NHS.

The new National Institute for Clinical Excellence said that at £24 per course, Relenza was too expensive for the health service.

Glaxo involved 6,000 patients in clinical trials, which appeared to show that the drug caused no adverse reaction. The side effects have emerged only in recent weeks.

Some 8,000 patients in Britain have used Relenza.

Side effects

Glaxo wrote to around 36,000 British GPs warning that Relenza can trigger "bronchospasm" - a temporary narrowing of the airways into the lungs.

The company said it could also lead to a "decline in respiratory function" among patients with asthma or chronic lung disease.

"These letters were sent out following some very rare adverse events that took place in America recently," said a Glaxo spokesman.

"The European regulatory authorities have asked us to ensure that any physicians treating vulnerable patients for influenza take special precautions when prescribing Relenza to them."

Patients who experience side effects are advised to stop using Relenza and seek medical advice.

Glaxo recommends that those with asthma or chronic lung disease have an inhaler to hand if they suffer breathing problems.

"We are talking about a small group of highly vulnerable patients. The letters are merely reminders that doctors should monitor them closely anyway," the spokesman added.

The drug is taken as a dry powder via an inhaler twice a day for five days.

If taken within two days of the first symptoms, it can shorten the length of the illness by 40%, as well as easing common flu symptoms.