This question is exercising the minds of drug companies and scientists alike. A report in this week's Lancet estimates there are 140,000 people with serious heart disease in the US caused by use of the painkiller Vioxx.

The arthritis drug was withdrawn in September after evidence emerged that it could cause heart problems. Drug safety expert David Graham and colleagues at the Centre for Drug Evaluation and Research found those taking Vioxx had a 34% higher chance of coronary heart disease than those on other painkillers.

British experts say up to half a million people in the UK may be affected.

"Signals of the problem were noted between 1999-2001, but nothing was done," says David Webb, professor of clinical pharmacology at Edinburgh University, "The companies involved should have taken some action to prevent this disaster."

Dr Graham agrees. "The US regulatory authority hasn't acted on behalf of public health, but corporate interests. It was aware of the scale of the problem in June 2000 but waited two years to do something. If the regulatory bodies remain the way they are, the same disaster will reoccur."

Drug industry expert and critic Charles Medawar believes the entire control system needs reviewing. "We need a thorough inquiry not only into this problem, but into the regulatory system that allowed it to happen."

The UK Medicines & Health Care Products Regulatory Agency defends the system, of which it is part, saying: "No matter how extensive the clinical trials in patients, certain adverse effects may not be detected until a very large number of people have received the medicine."

The agency is investigating the safety of similar painkillers and advises patients to contact their doctor. "This is one of the worst health disasters in the world, and it demonstrates the way drug safety is assessed. Pre-marketing assessment is inadequate to ensure safety," says Dr Graham.

"There should be a new, patient-centred [system], one that focuses drug control on the way in which drugs are actually used, and on the safety of people who use them," says Medawar.