We all have heard of the placebo effect, but even in the medicinal circles, relatively few have heard of the nocebo effect.

In fact, this phenomenon is an evil twin of the placebo effect.

The nocebo effect refers by definition to the worsening or the induction of symptoms induced by active or sham therapies. (1)

While the placebo effect causes positive results, which normally shouldn't have any effect (e.g., sugar pill), the nocebo effect gives totally opposite result in health terms, due to the patient's negative thoughts and worst expectations.

These both effects are very powerful and regardless of whether the patient and the doctor are aware of it or not, it plays a significant part of the healing process.

Physicians Ignore Both Placebo And Nocebo

A large number of physicians and experts are surprisingly ignorant of the placebo effect, although it works in their favor. And if they don't want to hear about the placebo effect, you can imagine what would be their reaction to nocebo effect. Luckily, there are always exceptions!

Drugs play their role, but it is very important to understand how the human mind processes information.

In the 1970s, doctors diagnosed a man with terminal stage of liver cancer and even told him he had just a few months to live. The patient really died in the predicted time but an autopsy showed the doctors had been wrong. There was a tiny tumor which had not spread at all.

Or for example, if you tell people a medical procedure will be extremely painful, they'll experience more pain than if you had kept the bad news to yourself.

In the same way, experiences of side effects within the placebo groups of drug trials show that when a doctor warns about the possible side effects of a medicine it is much more likely that the patient will experience those effects.

The nonverbal and verbal communications of physicians unintentionally contain large number of negative suggestions that may trigger a nocebo response. Therefore, this raises the important issue of how physicians can at the same time minimize nocebo‐related risks and obtain informed consent. (2)

The underlying mechanisms are, on one hand, neurobiological (role of endogenous opioids, cholecystokinin and dopamine) and, on the other hand, psychological (negative expectations and conditioning). As the placebo effects can modulate the outcome of a given therapy in a positive way, so can the nocebo effects but in a negative way. (3)

When Can The Nocebo Effect Occur?
  1. In direct interaction between physician and the patient, where each side has its own expectations.
The more the patient trusts the physician, the greater the nocebo effect will be.
  1. When expectations and general belief of patient are far from the therapeutic situation.
  2. When patients' beliefs and expectations are formed in certain groups.
We are more susceptible to negative thoughts and suggestions than to the positive ones.

A variety of scientific studies about the placebo and nocebo effects reveal that average efficiency of placebo is in 33% to 55% of cases.

On the other hand, the effective rate of the nocebo effect has astonishing 55% to 100%.

Unwanted Side Effects

The nocebo effect might even be powerful enough to kill. So, the same mind-body power that heals you can also harm you.


Those treated only with placebos often report ringing in the ears, vomiting, muscle weakness, fatigue, colds, memory and taste disturbances, and other symptoms that a sugar pill shouldn't trigger.

Even more interesting is that these nocebo complaints aren't random. They usually tend to arise as a result of the side effect warnings on the actual treatment or drug.