The manufacturers of an anti-HIV drug have recalled the product after admitting "human error" has led to higher-than-normal levels of a chemical that can cause cancer.
William Burns, chief executive of Roche's pharmaceutical division, said the impurity was caused by an interaction between two chemicals, one of them a cleaner, in a vessel in which the drug is made.
The drug Viracept, which has the generic name nelfinavir, has been recalled after tests on batches revealed high amounts of methane sulfonic acid ethyl ester.
The chemical is used in the drug normally but in smaller quantities, Roche said.
Mr Burns said the impurity is "low level", but the company believes it is unacceptable in its product.
Around 550 patients in the UK are thought to be on the drug, and they are now being urged to contact their doctor immediately.
Suspicions were raised after some patients reported a strange smell coming from their supplies, which come in powder and tablet form.
Viracept is a protease inhibitor - a class of drugs that helped to revolutionise HIV treatment in the 1990s.
It works by lowering the amount of virus in the body and slows the progression of the disease from HIV to Aids.
Roger Pebody, treatment adviser for the Terrence Higgins Trust, said the drug is not widely used in the UK now.
He said: "These people have probably been on the drug for several years and it has worked well for them, so they have had no reason to change it."
Terrence Higgins Trust
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