One of many such studies: https://pubmed.ncbi.nlm.nih.gov/19230461/ https://pubmed.ncbi.nlm.nih.gov/19230461/ Keep in mind this doesn't say that it's not effec

One of many such studies: https://pubmed.ncbi.nlm.nih.gov/19230461/ https://pubmed.ncbi.nlm.nih.gov/19230461/ Keep in mind this doesn't say that it's not effective. It's just not more so than the placebo effect, which is quite real even on physiological matters.