never take anything anyone gives you --doctor or no--without first reading up.
read up on:
1. expected effects
2. what it's supposed to do for you
3. possible side effects
4. what other things it can interact with
5. and for good measure, google something lik "[substance] has made my life hell" and see what turns up. there's forums for practically anything, including drugs, vitamins, etc.
just because someone took some courses and got a certificate and can now legally give you something doesn't mean they have the time or innate skill to always give the right thing, at the right time, to the right person, with the right instructions and info.
also: a pharmacist is a better bet for info than a doctor. pharmacys focus on one thing only: the prescribed substances.
doctors have a whole bunch of other concerns and often, are too busy to keep up to date on drug research. also, they receive little training on vitamins, nutrition, and so on.