Simply expecting a pill to harm you can make it harm you
The nocebo effect is the evil twin of the placebo effect: telling someone a harmless treatment might cause side effects makes some of them genuinely feel those side effects, with measurable symptoms and even real physiological changes. Trial patients given a sugar pill but warned about nausea or headaches report them at real rates. Physician Walter Kennedy coined the term in 1961, precisely to separate the patient's own expectation from anything actually in the remedy.