Most of us already know about the placebo effect. As part of medical studies, a control group is typically given an inert substance (usually a sugar pill) that provides a baseline to which researchers can compare the effectiveness of the new medicine being tested. The members of this group are told that the placebo is real—and surprisingly, they sometimes experience an actual improvement in their symptoms, simply because they expect that the medicine will make them feel better.

An opposite tendency—and one that has been largely overlooked by the research community—is the nocebo effect. The term nocebo (Latin nocēbō, “I shall harm”) was coined by Walter Kennedy in 1961 to denote the counterpart to the use of placebo (Latin placēbō, “I shall please”).

Put simply, it is the phenomenon in which inert substances or mere suggestions of substances actually bring about negative effects in a patient or research participant. Like the placebo effect, it is still poorly understood and thought to be brought about by a combination of Pavlovian conditioning and a reaction to expectations.

In one study, 50 people – who suffered from chronic back pain – were randomly divided in two groups:

– the group A:  patient were tested with a flexion test.  Patients were told beforehand that the test might cause some pain

– group B: patient were tested with a flexion leg test. Patient were not given any information beforehand

The group A reported a significantly higher amount of pain, despite enduring the exact same procedure of the Group B


In another study,  Mondaini and coworkers analyzed the role of nocebo on erectile disfunction in patients with prostatic disease.  They divided the patients in two groups:

  • group A: they administered the drug finasteride for relieving symptoms of prostate disease. The patient were informed about the possibility of erectile dysfunction before starting the treatment. The phrase used to inform group A patients was “. . . it may cause erectile dysfunction, decreased libido, problems of ejaculation but these are uncommon“.
  • group B. they administered the drug finasteride for relieving  symptoms of prostate disease. The patients were not informed of the possibility of erectile dysfunctions owing to the pharmacological therapy.

After 6 and 12 months of therapy, the patients were administered a questionnaire.  The incidence of reported erectile dysfunction was 43% in group A versus 15% of the group B.


The nocebo effect might even be powerful enough to kill. Reeves and coworkers described  an individual who experienced unusual negative effects while taking a placebo during a clinical drug trial. A 26-year-old male took 29 inert capsules (placebo sugar pills), believing he was overdosing on an antidepressant. Subsequently, he experienced hypotension requiring intravenous fluids to maintain an adequate blood pressure until the true nature of the capsules was revealed.  After it was revealed that they were sugar pills, the symptoms went away quickly.

It’s commonly thought that properly preparing a patient for pain—for example, “this might hurt quite a bit”—is the best way to minimize anxiety, so the patient knows what to expect. But one experiment analyzed showed that the very words used by a doctor before injecting radiographic substances affected the amount of pain experienced. The more frequently the words “sting,” “burn,” “hurt,” “bad” and “pain” were said, the more discomfort felt by patients. The researchers suggest that doctors reconsider conventional beliefs about pain management to avoid magnifying painful side effects.

One example of the nocebo effect was extracted from the Framingham Heart Study, which began in 1948 to identify common factors that contribute to cardiovascular disease. It began with 2,873 women and 2,336 men. Women who believed they were prone to heart disease were nearly four times as likely to die as women with similar risk factors – high blood pressure, excessive weight, high cholesterol – who didn’t believe.


In a classic nocebo experiment conducted in the early 1980s volunteers were told that a mild electrical current would pass through their head, and although no current was used, two-thirds of the volunteers complained of a headache after the experiment.

Evidence suggests that the symptoms of electromagnetic hypersensitivity are caused by the nocebo effect.

Nocebo Effect is almost like a negative self-fulfilling prophecy. Some people maintain that belief kills (e.g., “voodoo death” or “evil eye”)  a and belief heals (e.g., faith healing). That’s why I call nocebo effect also “voodoo effect“.

People get worse because they believe they’ll get worse.

Creating uncertainty (“this medication may help”, “let’s try this drug”), Emphasizing the negative (“you are high-risk patient”, “that always hurts a lot”), Focusing attention (“signal if you feel pain”, “are you feeling nervous?”), Ineffective negations (“you do not need to worry”) are a fuel for the nocebo effect.

Figuring out how to handle patients’ fears and anxieties, it turns out, can be just as difficult as fighting real diseases and infections. The surprising conclusion you might come to after learning about the nocebo effect?  What you don’t know can’t hurt you.




Häuser W, Hansen E, Enck P: Nocebo phenomena in medicine: their relevance in everyday clinical practice. Dtsch Arztebl Int 2012; 109(26): 459–65.

Elvira V Lang, Eric G Benotsch, Lauri J Fick, Susan Lutgendorf, Michael L Berbaum, Kevin S Berbaum, Henrietta Logan, David Spiegel. Adjunctive non-pharmacological analgesia for invasive medical procedures: a randomised trial. Lancet 2000; 355: 1486–90

Lancet 2000; 355: 1486–90

Reeves RR, Ladner ME, Hart RH, Burke RS. Nocebo effects with antidepressant clinical drug trial placebos. Gen Hosp Psychiatry. 2007 May-Jun;29(3):275

Mondaini N, Gontero P, Giubilei G, Lombardi G, Cai T, Gavazzi A, Bartoletti R.  Finasteride 5 mg and sexual side effects: how many of these are related to a nocebo phenomenon? J Sex Med. 2007 Nov;4(6):1708-12. Epub 2007 Jul 26.

Pfingsten M, Leibing E, Harter W, et al.: Fear-avoidance behavior and anticipation of pain in patients with chronic low back pain: a randomized controlled study. Pain Med 2001; 2: 259–66.

Buffy Eldridge-Thomas and G James RubinIdiopathic Environmental Intolerance Attributed to Electromagnetic Fields: A Content Analysis of British Newspaper Reports.PLoS One. 2013; 8(6): e65713.