Most British doctors say they've prescribed placebo treatments to harness the power of suggestion, a new study comparing ethics and acceptance shows.

Previous research suggests 20 per cent of Canadian doctors also use placebo treatments hoping to create a placebo effect that reassures patients or improves symptoms. Some medical authorities consider the use of placebos to be unethical.

Most doctors said they've used some kind of placebo treatment at least once. (iStock)

For the study in the journal PLOS One, researchers checked through the answers of 783 doctors who were surveyed online. The sample included 71 per cent of all doctors registered with the U.K.'s General Medical Council.

Among the respondents, 97 per cent said they'd used substances or methods that lack therapeutic value for the condition prescribed such as:

Antibiotics to fight bacteria for a suspected viral infection like a cold.

Vitamin C for cancer.

Non-essential examinations like blood tests or X-rays that are costly to the health-care system and may carry radiation risks.

For authorities to put their heads in the sand and pretend [placebo treatments] are not being given out is not helpful," said study author Jeremy Howick of Oxford University. "We need to think of ways to maximize the benefits of using placebos."

Half of those who prescribed pure placebos told patients that "this therapy has helped many other patients," a quarter told patients that the treatment promoted self-healing and a tenth were up front in telling the patient the treatment was a placebo. In the study, 12 per cent of respondents said they prescribed pure placebos such as sugar pills or saline injections without direct pharmacologically active ingredients.

Dr. Tony Calland, chair of the British Medical Association's Ethics Committee, said he was disappointed by the findings.

"Prescribing something that you know is of no value is not ethical."

Surveys in other countries suggested that anywhere from 17 per cent to 80 per cent of doctors prescribe placebos. A 2011 study by Dr. Amir Raz, a psychiatry professor at McGill University in Montreal, suggested about 20 per cent of psychiatrists and other specialists have prescribed placebos at least once.

At that time, Raz told CBC Radio's The Current that there are no formal policies on clinicians using placebos for their patients. The strong and in some cases long-term physiological effects of placebos can be as potent as drugs with pharmaceutical effects and do need judicious guidelines.