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, supplements or an unnecessary exam, according to a new survey.

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

The survey asked doctors if they had ever used a true placebo, like a sugar pill or dummy treatment not meant for the patient’s condition or a nonessential exam. Nearly all the doctors, 97 percent, reported using some kind of placebo treatment at least once. About 77 percent said they used some kind of placebo treatment every week.