The question about drugs causing mental health problems is a rather vexed one. Working in a mental health setting I see plenty of people with psychotic illnesses - it's now rare to find someone having their first episode of psychosis who doesn't use/abuse drugs-though acid is much less commonly used in the people I see than heroin/e's/speed. Partly that reflects drug use amongst young people (for it is predominently the under 23's who get their first episode of psychosis) and i guess there are a number of people who have a latent vulnerability to psychosis whose drug use "uncovers" an illness that might otherwise have lain dormant. However, I've also come across others who have started using hallucinogenics after they've started having their first psychotic symptoms (long before they come to psych attention) possibly as a form of self medication to assist in making sense of a world gone weird.
And it was used relatively recently (early 60's)in psychiatric settings (eg in individual psychotherapy) to facilitate the therapeutic process. - I know at least one now retired psychiatrist who used to take it along with his patient in the therapy setting... I was born too late man!!