Experts believe this may happen because you believe you're taking something that will make you feel better - in other words, it's a case of mind over matter. The phenomenon is often, for instance, seen in clinical trials, where some volunteers are given dummy pills instead of real pills.
In fact the evidence for using placebos is so compelling that one study suggests 50% of American doctors give dummy pills to their patients without them knowing.
But a new study suggests there's even more to it than the belief that you're taking a real medicine. Researchers from Harvard Medical School say tests show that even when people realise what they're taking is a placebo, the sugar pills still work.
Writing in the journal PLoS ONE, the researchers describe how almost twice as many people treated with dummy pills for irritable bowel syndrome (IBS) said they felt better than those who weren't given any pills. But they knew full well that what they were taking was a placebo.
"Not only did we make it absolutely clear that these pills had no active ingredient and were made from inert substances, but we actually had 'placebo' printed on the bottle,"' says lead researcher Professor Ted Kaptchuk.
The experts claim that just taking tablets - despite knowing there's nothing in them - may encourage the brain to start healing the body.
Do you think the mind is the best medicine?