LONDON (AP) -- 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.

Howick and colleagues used a Web-based survey and got 783 responses. The sample 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.

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.

Dr. Tony Calland, chairman 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," he said.

A previous study found about half of U.S. doctors regularly give their patients treatments that probably won't work without telling them, and the practice has been reported elsewhere, including Canada, Denmark and Switzerland. The American Medical Association says physicians may only use placebos if the patient is aware.

In 2011, the German Medical Association recommended doctors use fake pills and other placebo treatments more often and said patients didn't necessarily need to be told.

Some small studies have found dummy pills work even when patients are explicitly told what they're getting and others have documented the fake treatments can spark a biological effect in the body.

"For illnesses where there is no truly effective treatment, a placebo or alternative therapy is a fine thing to do," said Dr. Walter Brown, a clinical professor of psychiatry at Brown University and author of a recent book on placebos. He was not connected to the new study.

Brown said doctors weren't obligated to use the word "placebo" when prescribing the treatment. He said doctors should just be honest with patients and suggested they tell them the pill has no medication in it but might still somehow help.

Some patients found it hard to believe doctors would ever give them a placebo treatment, including pills or an unnecessary exam, without telling them. Alex Tellaie, who brought his son to a north London medical clinic, was doubtful his doctor would be dishonest.

"Doctors in this country don't do that," said Tellaie, 42, a private driver. "I trust the doctor to give my son what he needs."

Others said they would be willing to take a placebo, but only if they knew in advance.

"I wouldn't be too happy to take something without knowing what it is and what it might do," said Nick Christophi, a railway worker. "But if the doctor says there's a chance that it could work, I would probably be willing to give it a go."

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