Sometimes both placebo and nocebo effects are seen in the same study. The New England Journal of Medicine once recounted a test in which patients were injected with either a food allergen or harmless saline solution. The percentage experiencing a range of symptoms, from watering eyes to depression, was 27 percent in the allergen group and 24 percent in the saline group. An inert "neutralizing" injection in some cases the same saline solution that had produced the symptoms was equally effective in stopping reactions in both groups.

A placebo relieves symptoms of illness by creating an expectation of improvement, while a nocebo does harm by creating the opposite expectation, according to epidemiologist Robert A. Hahn of the Centers for Disease Control and Prevention. Nocebos are not used in medical research, but nocebo effects are quite common.

Research shows that pain is not merely a simple electrical nerve response. Perhaps surprisingly, there is relative independence of pain from the body's pathological processes. Many studies show that pain may be reduced with a placebo (a dummy medical treatment). It also can be psychologically induced through the use of a nocebo. Two researchers, Doctors A. Schweiger and A. Parducci, demonstrated this in a 1981 article in the Pavlovian Journal of Biological Science. Pain stimuli may also evoke differing behavioural responses according to age, prior psychological status, subjects' set (type of group), social context, and culture.