A placebo can be highly effective for pain — even when the patient knows it is a placebo, a new study found.

Portuguese researchers recruited 83 people with chronic back pain. They explained to them that a placebo was an inactive substance, like a sugar pill, that contained no medication. Then they randomly assigned the patients to either treatment as usual (in almost all cases, this was pain medication), or treatment plus the placebo. The pills were provided in a prescription medicine bottle marked “Placebo pills. Take 2 pills twice a day.” The study is in the journal Pain.

At the start and end of the three-week trial, patients filled out questionnaires describing pain intensity and degree of disability.

The group that got their regular treatment had an average 9 percent reduction in usual pain and a 16 percent reduction in maximum pain. But the placebo group averaged a 30 percent reduction in both usual and maximum pain. The placebo group also reported a 29 percent reduction in disability, while the usual treatment group reported none.