It takes belief to have a placebo or nocebo effect.

Our beliefs can make us more or less likely to believe something else. Long-term familiarity with a doctor may make a patient more likely to have a stronger placebo reaction due to that previous trust relationship. A doctor might also have pre-conceived bias towards or against a treatment. When a bias of knowledge exists then it also may take more evidence to change the person's opinion.

See for more information: Bayes and Belief How pre-review belief influences critical appraisal. St. Emlyns.

Disclaimer: Information shared for educational purposes within the guidelines of Fair Use.