Vagus Nerve Stimulation Improves Severe Depression by 70%, Major US Trial Finds
(www.sciencealert.com)
Honestly, look into pain killer studies. We still don't know how many of them work even though we know that they do work.
Tylenol (acetaminophen/paracetamol) is one of the most used painkillers and we still don't know the mechanism of action.