Everyone is aware of the placebo effect, in which a mock or inert intervention like a sugar pill in a clinical trial will nonetheless produce what seem like real effects to the people who receive it. The nocebo effect is the opposite: where people exposed to something harmless can experience apparent ill effects because they believe it is harmful.
A recent study found nocebo effects to be widespread with the COVID-19 vaccines: “the ‘nocebo effect’ accounted for about 76% of all common adverse reactions after the first dose and nearly 52% after the second dose.”
This is a reminder about the humility we need to maintain when interpreting our own medical experiences. Just because something came after something else doesn’t mean the first thing caused the second, and just because an effect seems psychologically or emotionally connected to a cause does not mean there is an empirical or causal connection.