For some psychological conditions there is also an effect from merely participating in a study. For example, if you have depression then the human interaction and feeling that someone cares about you has some therapeutic effect.

What about a "reverse" effect: https://en.wikipedia.org/wiki/Voodoo_death

The "reverse" effect would be a Nocebo. Which also exists. https://en.wikipedia.org/wiki/Nocebo

Thanks, that's a more accurate reverse effect to placebo.

I was thinking in a wider sense, negative effect of mind on health instead of positive. It's intriguing to what extent it's speculated the mind is able to affect the physical health.

There is scientific consensus that placebos have a subjective effect on symptoms, argued to be some unknown internal/mental effect. This is very widely held in the medical sciences community, and has more broad acceptance than most things held as a so-called consensus. It is not and has never been a lay person thing.

e.g. An article like https://www.health.harvard.edu/newsletter_article/the-power-... is not kooky or controversial, or the creation of a lay person. It's the accepted understanding of what's happening -- at least what is seemingly happening -- as viewed by medical experts.

As an aside, while placebo does come from Latin for "to please", you have it backwards. Placebos are to please either the patient or the participant, giving them a feeling that they're being treated/studied.