This is a good question and one that is hard to know. In theory you need to have some mechanism of action, but drugs do get approved without one, or more commonly with a wrong one. Quite a few of the drugs in the psychiatric field are little more than placebos-with-bad-side-effects that got lucky.<p>As far as I know nobody has ever been sued over a drug that didn’t work. From a purely legal perspective you would better selling a sugar pill than a real drug that worked and caused 1 in 100,000 people to drop dead.