The calcium channel antagonists are generally safe in therapeutic dosage, but severe side effects with elevated intake are increasingly described. Typical features include confusion, lethargy, hypotension, sinus node depression, and cardiac conduction defects. Even if patients are stable on presentation, this does not preclude the possible late development of adverse events from the long-acting formulations of calcium channel blockers. A case of toxic overdose with 1440 mg of slow-release diltiazem is presented; this patient was stable on presentation, but rapidly became hemodynamically unstable, requiring treatment with intravenous calcium, temporary pacemaker, inotropic support and mechanical ventilation with a successful outcome. A concise review of the therapeutic considerations is provided.