What Is the Nocebo Effect?

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The nocebo effect, also known as the nocebo response, happens when a person's negative expectations of treatment lead to negative side effects. An example of a nocebo response would be a person expecting that the medication will cause negative side effects and then having those unpleasant side effects even though the medication that they are taking is actually an inert substance.

People tend to be more familiar with the nocebo effect's counterpart, the placebo effect. In medical studies, a group of participants known as the control group will be given a sham treatment, often a sugar pill, that they believe is the real treatment. As a result of believing that the treatment is real, participants will sometimes have positive results such as feeling better or an improvement in their symptoms.

In the placebo effect, people experience positive effects as a result of their positive expectations. The nocebo effect is its opposing counterpart. The suggestion that there might be side effects leads people to actually experience those side effects, even when the treatment isn’t real.

Where the placebo effect makes a person feel better beyond the actual therapeutic effects of the treatment, a nocebo effect causes people to feel worse.


Adverse effects can vary from one individual to the next, cannot be attributed to the pharmacological activity of the drug, and are not dependent upon the dose.

Nocebo effect symptoms often include:

  • Drowsiness
  • Nausea
  • Dizziness
  • Difficulty concentrating
  • Headache
  • Insomnia
  • Fatigue
  • Itching
  • Bloating
  • Stomach pains
  • Loss of appetite

It is important to note that these side effects are unique to the individual and may be influenced by a variety of factors.


Some examples of the nocebo effect that have been observed in research:

Headaches: One study found that nocebo effects were prevalent in studies focusing on headache treatment and prevention. Because participants who had a nocebo response were also more likely to dropout, it was more likely to also affect the interpretation of clinical trials.

Pain: The nocebo effect can also play a role in how people perceive pain. Clinical trials focus on the nocebo effect in pain treatment also showed that those who had negative nocebo responses were more likely to quit the trial.

Drug Response: Another study found that a high number of people (including health consumers and medical professionals) have negative attitudes toward the efficacy of generic drugs. The study found that people may be more likely to experience more side effects in response to generic drugs due to these negative expectations.

Such effects are not confined to research labs or clinical trials. The nocebo effect has the potential to cause serious consequences in real-world medical settings where people receive care.

That said, some generics do indeed cause a greater number of side effects or lack the efficacy of the brand name because of variability in formulation and ingredients, so it may be hard to differentiate between actual, real adverse effects and the nocebo effect.


The exact explanations for the nocebo effect are not well understood, but researchers have found that a few factors influence how likely it is to occur. These include:

  • How doctors and nurses talk about the effects of treatment: Suggestions that express uncertainty, using technical jargon, ambiguous statements, and emphasizing the negative are all practices that can increase the nocebo effect.
  • Cost: Research has shown that the cost of medication can influence perceptions of how effective it is.
  • Patient expectations: Research has shown that just announcing that a drug will be given can lead to side effects, even if the drug is not actually administered.
  • Past experience with treatment: People who have had negative side effects in the past from similar treatments may expect that future treatment will have the same negative side effects.


Researchers have found that the phenomenon is both surprisingly common and serious. It can have an important impact on both research and treatment, suggesting that it is something that requires consideration from scientists and medical practitioners.

Informed Consent

This can create an ethical dilemma for doctors, nurses, psychologists, and other healthcare professionals. Telling people about all of the potential downsides of treatment may cause those negative effects to happen or make them worse, but not telling people about these effects would violate rules about informed consent.

Informed consent requires that doctors and researchers inform patients and subjects of any possible adverse side effects when taking a medication or undergoing treatment. Researchers suggest, however, that presenting people with a list of potential side effects can increase the likelihood of a nocebo response. In other words, describing what might happen actually has the effect of changing what people experience.

Treatment Adherence

The fact that the nocebo effect causes negative outcomes can also have an effect on treatment compliance and research dropout rates. People may be less likely to continue taking treatment if they are having unpleasant side effects, regardless of the fact that those side effects are actually related to expectations and not to the effects of the treatment itself. Participants in research studies may also be more likely to drop out of the study if they experience nocebo effects.

Worsening of Negative Side Effects

Telling people that they might have side effects from a particular treatment may actually lead to nocebo responses that make these negative outcomes worse. In one study, for example, some participants were told that a flexibility test might cause pain while others were not told this. After the test, those who had been told that the test might be painful reported experiencing significantly higher levels of pain than those in the other group.

Because of this, researchers caution that doctors and therapists should consider the powerful impact that cognitive processes and expectations can have on the experience of pain. People who fear that treatment will cause pain may end up avoiding treatment.

A Word From Verywell

The nocebo effect can have a serious impact on how people feel after a treatment, so healthcare professionals should use caution when presenting treatment options. This phenomenon is also a powerful example of how negative thinking can influence your health and well-being. When you are undergoing treatment or taking a medication, it may be more helpful to focus on the potential positive benefits rather than worrying about the possible negative side effects.

8 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Kendra Cherry, MSEd
Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."