https://www.health.harvard.edu/mental-health/the-power-of-the-placebo-effect

A study led by Kaptchuk and published in Science Translational Medicine explored this by testing how people reacted to migraine pain medication. One group took a migraine drug labeled with the drug’s name, another took a placebo labeled “placebo,” and a third group took nothing. The researchers discovered that the placebo was 50% as effective as the real drug to reduce pain after a migraine attack.

The researchers speculated that a driving force beyond this reaction was the simple act of taking a pill. “People associate the ritual of taking medicine as a positive healing effect,” says Kaptchuk. “Even if they know it’s not medicine, the action itself can stimulate the brain into thinking the body is being healed.”

Placebo works even when the subject knows it’s a placebo.