A drug that is taken by hundreds of HIV-positive patients in Britain was recalled across Europe yesterday after the medicine was found to contain a toxic substance that can cause cancer.

Patients who take Viracept, an antiretroviral drug, should contact their doctors immediately, health officials advised.

The Medicines and Health-care products Regulatory Agency (MHRA) and European regulators said that batches of Viracept across the European Union had been tainted by a “genotoxic”, or potentially cancer-causing chemical.

The drug’s Swiss manufacturer, Roche, said that the drug had been contaminated by methanesulfonic acid ethyl ester, which is classified as carci-nogenic by the International Agency for Research on Cancer. The recall is likely to cost the company tens of millions of pounds.

Viracept, which is used to treat HIV in its early stages and is taken by about 19,000 patients in Europe, belongs to a class of drugs called protease inhibitors which can slow down the spread of the virus in the body and reduce the risk of patients developing diseases that are caused by Aids.

“In the interest of patient safety, Roche has decided to recall all batches of Viracept tablets and powder,” the company said, adding that the US, Canada and Japan were not affected by the warning.

In its statement, the MHRA said that it was “alerting health professionals tonight concerning a contamination with a genotoxic substance affecting the production of all batches of the medicine Viracept.

“The MHRA, in conjunction with the European Medicines Agency (EMEA) and Roche, has issued a drug alert to recall this medicine from the market, to minimise the risk to patients. Patients prescribed Viracept should contact their doctor immediately. They will have to change to another appropriate medicine for their condition.”

“There is a problem in manufacturing,” said a spokesman for the EMEA. “The level of risk to patients resulting from this contamination is difficult to measure, and is currently under further evaluation.”

Viracept, whose generic name is nelfinavir, is a antiretroviral agent used by HIV patients across the world as part of a variety of drugs to control the virus.

The drug works by lowering the level of virus in the body (the viral load) and slows the progression of the disease from HIV to Aids. It was developed by Agouron Pharmaceuticals in the early 1990s and became popular because it is easy to take, with fewer side-effects than other drugs of its type.

Viracept is sold in the United States by Pfizer and to the rest of the world by Roche. In 2003 Roche agreed to reduce its price for sale in sub-Saharan Africa.

William Burns, the head of Roche’s pharmaceutical division, said that the contamination was the result of human error. “There is a low level impurity here which we believe is unacceptable to be in our product,” he said. “We are talking low parts per million, we are talking in extraordinarily small quantities.”

Roger Pebody, treatment adviser for the Terrence Hig-gins Trust, said that Viracept was in one of the older classes of HIV drugs and was not widely used in Britain.

But he added that missing only one dose could impede the effectiveness of the treatment, and that patients should contact their doctors immediately.

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