But to answer your question posed in the title, it's the Hippocratic oath.

> More subtly, doctors can fudge the language by asking patients if they consent to something like “try different medicines, including placebos” to retain the power of self-fulfilling improvement.

Unless your name is Charlie Kelly [1] - that ain't going to work.

[1] https://www.imdb.com/title/tt2999352