A nocebo is the opposite of a placebo. It is an inert substance--or even a real medication--that causes negative side effects or symptoms in a patient simply because it is what they are expecting from it.
Also referred to in the phrase "the nocebo effect," when the expectation of a negative outcome (symptom) that produces a real negative result.
As an example, people who believe that they suffer from Electromagnetic Hypersensitivity will experience real symptoms by imagining that exposure to electricity is causing them harm. In this case, it is electricity that is the nocebo.
Etymology: Coined in 1961 as a companion to the word "placebo" which dates back to the late 1700s. Latin, literally translating into "I will be harmful." Related: English word nocent meaning "harmful." From Latin nocēre meaning "to harm."