What is the MOST common adverse effect of systemic antibiotic therapy?

Correct answer: C. Gastrointestinal upset — nausea, vomiting and diarrhea

• A. Stevens–Johnson syndrome is a severe, life-threatening mucocutaneous reaction with widespread epidermal detachment. It is a genuine risk with sulfonamides and some other agents, but it is RARE.
• B. Anaphylaxis is the most FEARED adverse effect but is uncommon; the question asks for the most COMMON.
• C. CORRECT. GASTROINTESTINAL DISTURBANCE — nausea, vomiting, abdominal cramping and diarrhea — is by a wide margin the most common adverse effect of antibiotics. It arises from direct mucosal irritation and from disruption of normal gut flora. It is dose-related, generally mild, and the most frequent reason patients discontinue therapy prematurely.
• D. Nephrotoxicity is characteristic of specific classes — aminoglycosides and vancomycin — and is not a general or common antibiotic effect.

Note: Counsel patients to take antibiotics with food where the agent permits, to complete the full course, and to report severe or persistent watery diarrhea — which may signal CLOSTRIDIOIDES DIFFICILE colitis, most classically associated with CLINDAMYCIN but possible with any antibiotic.