Assuming you’re in the US.

From Prescription Drug Advertising: Questions and Answers on the FDA’s website.

Who should I tell if I think that a prescription drug ad violates the law?

Contact FDA’s Office of Prescription Drug Promotion (OPDP) about prescription drug ads you believe violate the law by being false, misleading, or lacking in “fair balance”. Consumers can call OPDP at 301–796-1200. Or, consumers may submit their complaint in written form to OPDP at:

Food and Drug Administration
Center for Drug Evaluation and Research
Office of Prescription Drug Promotion
5901-B Ammendale Road
Beltsville, MD 20705–1266

Just curious as to the details… what ad was it? How do you think it violated the law?