A large number of U.S. doctors admit that they regularly prescribe placebo drugs rather than the real thing to patients and find nothing wrong with the practice, a new study says.

A confidential survey found that half of internal medicine physicians and rheumatologists prescribe placebos on a regular basis.

Sixty-two per cent of the survey's respondents found it to be an ethically acceptable practice.

The most commonly prescribed placebos were over-the-counter painkillers and vitamins. However, some doctors also reported prescribing antibiotics and sedatives for their placebo effect.

Only three per cent of doctors said they prescribed sugar pills.

"Recommending relatively innocuous treatments such as vitamins or over-the-counter analgesics to promote positive expectations might not raise serious concerns about detrimental effects to patients' welfare," the authors wrote. "Prescribing antibiotics and sedatives when they are not medically indicated, however, could have potentially important adverse consequences for both patients and public health."

The researchers also found that the majority of doctors did not explicitly tell their patients that they were prescribing a placebo.

About 68 per cent of respondents described the treatment as "a potentially beneficial medicine or treatment not typically used for their condition."

Further research is needed to examine the circumstances in which placebos are used, the authors concluded.

The survey was conducted by researchers at the National Institutes of Health, Harvard Medical School and the University of Chicago. The findings are published at bmj.com.

The findings support other studies, which have shown that many doctors prescribe placebo treatments at least once a year.

The placebo effect refers to the medical benefits some patients receive after undergoing treatments that have no known benefit.

"The placebo effect is often explained as the result of positive expectation, belief, or hope in patients derived from the clinical encounter," the authors wrote.

Prescribing placebos is a controversial topic within the medical community. Many doctors feel that prescribing placebos "involves deception and therefore violates patients' autonomy and informed consent," the authors wrote.

However, advocates of placebo treatment say it can be an effective treatment for chronic conditions.

Internists and rheumatologists treat a number of debilitating chronic conditions that are difficult to manage with drugs alone.