Doctors were yesterday advised to switch all patients taking the painkillers known as Cox-2 inhibitors on to other drugs following revelations that they increase the risk of heart attack.

At least a million people are thought to have been on the drugs which were widely prescribed for arthritis before the first of the two market leaders, Vioxx, was withdrawn by the manufacturer Merck at the end of September. Four days ago Pfizer, which makes Celebrex, said it too had new data showing an increased heart attack risk.

Yesterday the UK licensing body, the Medicines and Healthcare Products Regulatory Authority (MHRA) wrote to all doctors instructing them to take their patients off all Cox-2 inhibitors. The alternatives are other non-steroidal anti-inflammatories, including paracetamol and ibuprofen.

The MHRA says this is "interim advice". Although it is charged with ensuring patient safety, it has not yet been shown data on the risks by Pfizer.

An earlier statement from the MHRA, on December 17, made it clear that the agency has been seeking details from the company without success. "We do not yet have full access to details of the trials - the only information we have has been extracted from the website of the manufacturer ... We have urgently requested access to the new study information."

Because Merck has withdrawn Vioxx worldwide, it will be under no obligation to pass any data about its trials.

It came under heavy criticism at the time of the voluntary withdrawal, however, for not having faced up to the problems with its drug early enough. Doctors writing in the Lancet said in November that it had enough information to pull the drug four years earlier.