Final answer:
The best references for looking up the adverse reactions of Zithromax are the Physician's Desk Reference (PDR) and the Merck Manual, which are both comprehensive resources used by healthcare professionals.
Step-by-step explanation:
The reference most appropriate for looking up the adverse reactions of Zithromax, which is a brand name for the antibiotic azithromycin, would be the Physician's Desk Reference (PDR). The PDR is a comprehensive book that provides information on prescription drugs, including their uses, dosing, contraindications, and potential side effects or adverse reactions. It is widely used by healthcare professionals. Another reliable medical reference that contains detailed information on drugs, including their adverse reactions, is the Merck Manual. While a Journal of Medicine might occasionally publish studies or reports on drug reactions, it is not as immediately useful as a direct reference, and the Encyclopedia Britannica would not have the detailed, drug-specific information necessary for a healthcare context.