Activated charcoal, having the capacity to absorb substances with its porous surface, has been used in intoxication treatment for nearly 200 years. Although live-saving, occasionally, it can lead to complications. Because of the risk of perforation during activated charcoal therapy, the integrity of the gastrointestinal tract should be checked after the procedure. In this case report, a 27-year patient, who received activated charcoal therapy after diclofenac intoxication developed duodenal ulcer perforation and charcoal peritonitis. The present case constitutes the first report of duodenal ulcer perforation after activated charcoal therapy. It should be remembered that activated charcoal, which is widely used in intoxication treatment, may cause gastrointestinal system perforation, peritonitis, adhesion, abscess formation, organ loss within the abdomen, and prolonged hospitalization. Key Words: Activated charcoal, Intoxication, Duodenal ulcer perforation.