You’ve probably heard of the placebo effect, the way in which fake treatments can improve symptoms. But you may not appreciate how strong this effect is. The mind is surprisingly powerful.

Two recent reports offer striking examples of the power of placebo. In one, orthopedic surgeon Bruce Moseley, team physician for the Houston Rockets, evaluated the effectiveness of an operation commonly used to treat the pain of arthritic knees.

In this surgery, doctors scrape away rough areas in the knee’s cartilage. It is widely reported to be effective, and as many as 400,000 such surgeries are performed each year. The benefits from this procedure may be largely due to the placebo effect, according to Moseley’s preliminary research.

In the study of 10 patients, five were given fake surgery, just little incisions over the knee. The others were given the actual surgery. Two years later, interviews found that pain and swelling were reduced just as much in the placebo group as in the group that received the real surgery!

Four out of the five participants who had the fake surgery said it was so helpful they’d gladly recommend it to a friend. A larger study of 180 patients is following up on these results. If it also finds that the surgery is no better than placebo, it’s going to be hard to justify continuing to perform this surgery.

Another study found placebo benefits from heart pacemakers placed in people with a serious heart illness called obstructive hypertrophic cardiomyopathy. In this three-month, double-blind study, each participant had a pacemaker installed. In one group the pacemakers were turned on; and in the other, they weren’t. The results, published in the American Journal of Cardiology, showed the same benefit in both groups. Non-functioning pacemakers made patients feel just as improved as functioning ones. Patients experienced significant improvement in symptoms such as chest pain, shortness of breath, and heart palpitations.

In addition, objective measurements of heart function showed considerable improvement in the placebo group. When researchers turned on the pacemakers, these effects were a bit greater. However, the improvement in subjective symptoms was identical in the two groups!

Impressive as these studies are, they are nothing new. Researchers have grown increasingly respectful of the placebo effect during the past few decades. Placebos have been found to affect just about any measurable aspect of physical function, and they can remain effective for months or even years.

Without proper studies, we don’t know whether an herb that has been used for centuries or an in-vogue medical procedure works. This is why double-blind studies are so important, and why we focus.