Which of the following is a side effect of activated charcoal?

• A. Rash and urticaria
• B. Headache
• C. Nausea, vomiting, constipation, black stool
• D. Dizziness

Answer: (C)

Activated charcoal works by adsorbing toxins in the gastrointestinal tract to limit absorption, so its side effects are mainly gastrointestinal. The combination of nausea, vomiting, constipation, and black stool is classic for this therapy: nausea and vomiting result from irritation of the stomach, charcoal can slow gut motility leading to constipation, and the charcoal itself darkens the stool giving a black color. Black stool is an expected and harmless sign of the treatment. Skin reactions like rash or urticaria and symptoms such as dizziness or headache are not typical, making the GI effects the best answer.