Diphenhydramine (Benadryl) is an over-the-counter antihistamine used to treat allergies, colds, motion sickness, and insomnia. Safe doses range from 25–50 mg every 4–6 hours, but misuse can cause severe side effects like hallucinations, delirium, heart issues, seizures, and even death. At high doses, users report disturbing hallucinations, including shadowy figures like the "Hat Man." Always follow dosing instructions and use responsibly to avoid serious health risks.