LONDON — More than three-quarters of British doctors prescribe a treatment they know probably won't work at least once a week, like low-dose drugs, vitamins, nutritional supplements or an unnecessary exam, according to a new survey.

This use of placebo treatments directly contradicts advice from the British Medical Association, which deems them unethical.

The researchers say the findings reveal a common practice among doctors and should be used to change official guidance about using placebos. The surveyed doctors said they prescribed them to induce a "placebo effect," to reassure patients or because patients pushed for a treatment.

"For authorities to put their heads in the sand and pretend (placebo treatments) are not being given out is not helpful," said Jeremy Howick of Oxford University, one of the authors of the study, which was published online Wednesday in the journal PLoS One. "We need to think of ways to maximize the benefits of using placebos," he said.

The survey sample of 783 responses was drawn from a list that included 71 percent of all doctors registered with the General Medical Council, the governing body for doctors in the U.K.

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The survey asked doctors if they had ever used a true placebo, like a sugar pill or another kind of dummy treatment such as a drug not meant for the patient's condition or a non-essential examination including blood tests and X-rays. Nearly all of the doctors — 97 percent — reported having used some kind of placebo treatment at least once, while 12 percent reported having used a fake pill.

About 77 percent of doctors said they used some kind of placebo treatment every week; more than 80 percent of them said their use in some circumstances was ethical.

The "placebo effect" treatments included unnecessary physical exams, joint injections, physical therapy, peppermint pills for a sore throat and antibiotics for infections where they would not be effective.