Fifty people die each year and hundreds more suffer serious side-effects as the result of taking powerful tranquillisers prescribed by the NHS, The Times has learnt.

Data from the medicine watchdog’s own reporting scheme suggests that clozapine, a drug taken by schizophrenia patients, has been linked to 950 deaths since being licensed in 1990 — equivalent to nearly one fatality a week.

Recent figures record the deaths of 55 people taking the medication in four months, from October to January, despite the drug being prescribed to relatively few people.

The anti-psychotic drug is used as a treatment of last resort for schizophrenia patients but appears to increase the chances of having a heart attack or stroke, or of suffering other long-term health problems that are not adequately monitored by current checks, campaigners say. More than 11,600 adverse reactions linked to clozapine have been reported in 19 years, according to the Yellow Card reporting scheme of the Medicines and Healthcare products Regulatory Agency (MHRA), which is designed to flag up potentially harmful side-effects for licensed drugs.

Mind, the mental health charity, which analysed the figures, said that the number of deaths associated with the drug was “chilling”. The MHRA promised to look into the charity’s concerns.

The warning comes as the National Institute for Health and Clinical Excellence (NICE) is due to publish guidelines tomorrow on the treatment of schizophrenia, which affects about one in every 100 people in Britain. The watchdog will recommend that oral anti-psychotic medications should be offered to people with newly diagnosed schizophrenia, although patients should take clozapine only after trying at least two other drugs.

The drug, also known by the brand names Clozaril, Denzapine and Zaponex, was prescribed on 7,000 occasions last year — many of which will have been repeat prescriptions — to help to control delusions and hallucinations.

People who take clozapine must be given regular blood monitoring for the risk of a potentially fatal blood disorder known as agranulocytosis, but at present there is no mandatory requirement to monitor their health in other ways.

NICE will recommend tomorrow that GPs and other primary healthcare professionals should monitor the physical health of people with schizophrenia at least once a year, with a focus on their risks of heart disease.

Alison Cobb, senior policy officer at Mind, said that the drug could be effective for some patients and acknowledged that people with schizophrenia had a higher risk of death than the general population. “There are health risks and side-effects associated with all anti-psychotic medications but it is chilling to see this number of deaths associated with clozapine and the month-on-month increases,” she said. “The figures refer to heart disease, strokes, cancer and other long-term health problems that cannot be explained simply by suicides or an increased overall risk of death.

“We want to see the use of clozapine properly reviewed and for people taking it to be offered the full range of health checks and alternative treatment options that might mitigate any harmful effects.”

A study published in the British Journal of Psychiatry last month concluded that use of clozapine in patients with severe mental illness was associated with a significantly increased risk of death compared with the general population. However, the author of the study, David Taylor, the chief pharmacist at Maudsley Hospital, South London, defended the use of the drug, which he said had “transformed the lives of thousands of people around the world”.

“Most of the deaths reported could not be said to be a result of clozapine treatment; none could be definitively linked to its use,” he said. “Clozapine, it is widely agreed, is under-used in this country and in practice its use is delayed for much longer than NICE currently recommend.

“The safety of clozapine has been continuously monitored since its launch in 1990,” he added. “In fact, no other drug has been as closely monitored. Close and intensive physical monitoring should be mandatory for all people with severe mental illness, regardless of treatment.”

In a statement, the MHRA added: “We are aware of the particular concerns of Mind in relation to clozapine and have already indicated to them that we will look further into the concerns they have raised and consider whether the trends in reporting through the Yellow Card scheme raise any new issues that merit update to the product information or indeed require reminders to prescribers about the need for close monitoring of patients to optimise safe use."

Simon Hough was diagnosed with schizophrenia 14 years ago and still manages his condition with medication. Fellow patients warned him off clozapine, after those taking it compared the effects to “like coming off cocaine or heroin”.

“They described feeling really low and lethargic, and the side-effects such as weight gain and dry mouth were so much worse than other drugs,” he said. “One guy I knew was average weight, about 10 or 11 stone, and he just ballooned up to about 18 or 19 stone when he started taking the drug. That’s got to have some impact on your long-term health.”

Mr Hough, 42, who now works for Making Space, a mental health charity in Northwich, Cheshire, took part in the CUtLASS study, a large-scale trial comparing the effects of modern antipsychotic drugs with those of older treatments.

“I was offered clozapine but after all that I’d heard about it I preferred to be in the group that took quetiapine [an alternative drug] instead.”