A nocebo is a substance which is not pharmacologically active, but which causes a patient to experience unpleasant or harmful reactions when he or she takes it. This is in contrast to a placebo, an inert substance which may cause a patient to feel better when he or she consumes it. Both the placebo and nocebo effects are caused by the power of suggestion, with the patient thinking that he or she will experience something by taking the drug, causing an effect to manifest.
People use the term “nocebo” to talk specifically about inert preparations which make people feel worse after they take them. The term means “I will harm,” and is a play on plaebo, which means “I will please.” When a patient takes a nocebo and he or she is told that unpleasant side effects may be experienced, these symptoms will manifest, even though there is nothing in the drug which could cause such side effects.
Sometimes, this term may also be used to describe a drug which is pharmacologically active, and also causes negative side effects. This usage of the term is not widely accepted, because the key difference between active and inert drugs is that an active drug is at least theoretically providing some benefit to the patient, even if the patient has some bad side effects as a result of taking the drug, while a true nocebo offers nothing to the patient.
Historically, placebos have been given to patients who demand some sort of medical treatment from their doctors, and no medication would really be effective. Placebos are also used in drug trials, with some patients receiving a placebo, while others receive the real drug. If patients respond differently to the two compounds, it suggests that the active drug is really effective. In placebo trials, the expected response to the placebo is nothing, because the drug contains nothing which could cause a change in the patient's condition, although many people on placebos report improvement in their conditions. Sometimes a nocebo effect emerges, and the patients on the fake drug experience bad side effects because they expect them to happen.
The power of suggestion can be amazingly effective. The nocebo effect has also been observed with some folk rituals and medicine. When people expect something to be harmful, they may react as though it was, even if it was inactive. For example, someone bitten by a harmless spider might experience symptoms of poisonous spider bite if he or she was told the spider was venomous.