A warning is to be issued about the widely used tranquilliser Ativan, which has been shown to have a rare but potentially fatal impact on patients' breathing.
Ativan, one of the benzodiazepine-class drugs taken by more than one million Britons each year, is prescribed for people suffering from anxiety, panic attacks and resulting insomnia.
Campaigners have accused the drugs companies for years of withholding information on the addictiveness of benzodiazepines, as well as their long-term side effects.
Earlier this year, it was discovered that Ativan's American makers, Wyeth, had told Canadian regulators the normal dose of the drug carried the rare, potentially fatal risk of causing respiratory depression. It can affect the part of the brain that controls the respiratory system.
Patients affected could take in too little oxygen temporarily, or even die, say doctors.
The Canadians accordingly changed their data sheet on the drug to tell prescribers the different adverse reactions that the drug, also known as lorazepam, can induce. They warned that initially patients should not be given more than one week's prescription of the drug before again seeing a doctor.
The Observer has learnt that Wyeth met the UK regulators, the Medicines and Healthcare Products Regulatory Agency (MHRA), earlier this year to warn it of the risk. The agency approved changes to patient leaflets issued with the tablets last September, but decided no public warning was needed.
Wyeth is now preparing new leaflets and data sheets to tell patients of the danger.
However, doctors urged patients yesterday not to stop taking the drug all at once because this can be highly dangerous. They said anyone worried should talk to their GP about the possibility of slowly reducing their dosage under supervision.
The news comes as drug firms face a challenge from Parliament to hand over papers on benzodiazepines. Phil Woolas, Deputy Leader of the Commons, believes they show that the companies have suppressed information on the addictiveness of tranquillisers.
Wyeth and Roche, which makes Valium and Mogadon, were accused last week of withholding evidence that their products were more addictive than their prescription guidelines said.
Woolas, a trustee of the anti-benzodiazepines Beat the Benzos campaign, told the Commons Health Select Committee he was convinced that the papers would prove this claim.
Benzodiazepines are prescribed drugs, which carry a risk of high dependency. It is common for patients to spend 12 months trying to withdraw from them. Although they are highly effective in the short term for treating symptoms of anxiety and insomnia, the effect wears off.
Wyeth said in a statement on Ativan. 'An additional warning relating to risk of potentially fatal respiratory depression with benzodiazepines, including lorazepam, was submitted to the MHRA earlier this year, and approved in September.'
The company declined to say what led it to issue the new warning, or whether it would change its advice to doctors on which patients might be at risk.
Heather Ashton, professor of psycho-pharmacology at Newcastle University, said: 'Most people don't realise how potent Ativan is, far stronger than Valium. It gets into the brain very quickly and it gets into the respiratory centre at the base of the brain, which is why it can affect breathing.'
