The study suggests doctors are underestimating the importance of side effects and should inquire more about them.

But mental health charities say that cost is the real reason for schizophrenics having to put up with drugs which cause them harmful side effects.

They say modern drugs which have fewer side effects are much more expensive than older treatments.

According to the survey, psychiatrists are almost a third more likely than patients to think side effects are a major reason for failure to take medication.

But they underestimate the extent to which patients suffer from them.

Sixty-six per cent of psychiatrists thought side effects were a major reason for patients failing to take medication, compared with 48% of patients.

However, only 63% of psychiatrists said their patients suffered from muscle spasms, facial tics and uncontrollable restlessness - compared with 87% of patients.

Impotence

Mental health workers believe between 29 and 37% of their patients suffer sexual side effects, such as impotence and reduced libido from antipsychotic medication.

Many schizophrenic patients find old-style drugs have strong side effects

This compares with 47% of patients.

The study, based on interviews with over 1,300 patients and health workers in six European and North American countries, was conducted by Dr Jonathan Hellewell, a consultant psychiatrist from the Trafford General Hospital in Manchester.

It is backed by drug company Zeneca.

Dr Hellewell says side effects are most common with older drugs.

He adds that many patients are reluctant to talk about side effects and thinks mental health workers should be more encouraging.

Important factor

He said: "The results [of the study] certainly show that the side effects of antipsychotic drugs are an important factor in a patient's decision to discontinue medication.

"And yet there is the suggestion that we as psychiatrists perhaps do not enquire about side effects quite as much as we might."

The results of the study, Impact of anti-psychotic medication: attitudes and perceptions study, will be presented at a psychiatrists' conference in Paris at the end of the month.

Stigma

The National Schizophrenia Fellowship (NSF) says some doctors are continuing to prescribe older drugs, despite the side effects.

It believes mental health patients are being denied the most appropriate treatment for their illness because of cost.

NSF chief executive Cliff Prior said: "Everyone should have access to the best possible medication, which has fewer side effects.

"NSF's own research points to a reluctance by doctors to prescribe the newer medications which can cost over £5 per day per patient compared to £0.08 per day per patient for the older medications."

Jonathan Zito: killed by community care patient

The issue of failure to take medication is being discussed by the government's new expert team on mental health.

Despite opposition from mental health groups, the government is thought to be pressing ahead with plans to introduce community treatment orders, under which community care patients would be forced to take their drugs.