And before that, you have to quiz him on whether the drug make sense at all, and if there are other and better alternative treatments. You should also do everything to educate your self on he drugs you are taking. Read the label on the prescription, and any brochure or printout the pharmacist gives you. You can ask for the package insert for the drug, which is a reprint of what goes into the Physician's Desk Reference. You can usually find a good drug reference in the public library, or consider buying a volume or two on your own.
Consumer Reports publishes a good book on the subject.
Always be sure to get the latest edition, as drug information is being updated all the time. Let him or her know you want to play a role in deciding what medications to take, and you want full details about the drug. In other words, let your doctor know you aren't going to be automatically gobbling pills, and want them only when the benefits are worth the risks. The pharmacist can save your life and prevent medication errors. So select the pharmacist as you select a professional, not on the basis of price only while ignoring quality.