Antihistamine Overdose: Symptoms, Risks, and What to Do

When someone takes too much of an antihistamine, a medication used to block histamine and treat allergies, colds, or sleep issues. Also known as H1 blocker, it can cause serious, even life-threatening reactions if taken in excess. Antihistamines like diphenhydramine (Benadryl), doxylamine, and promethazine are common in OTC sleep aids and cold meds. People often think they’re harmless because they’re available without a prescription—but that’s not true. Taking just a few extra pills can lead to antihistamine overdose, a toxic reaction from excessive intake, especially in kids or older adults.

What happens when you overdose? Your heart races, your mouth goes dry, your vision blurs, and you might start hallucinating. Some people get dangerously confused, stop urinating, or slip into a coma. These aren’t rare side effects—they’re documented outcomes. A 2021 study in the Journal of Medical Toxicology found that over 50% of antihistamine overdose cases involved people under 20 or over 65. The worst cases come from products like Benadryl Nighttime or Unisom, where people take multiple doses thinking, "It didn’t work last time." But your body doesn’t work that way. Too much blocks acetylcholine, and that’s what causes the nervous system to go haywire.

It’s not just about pills. Mixing antihistamines with alcohol, opioids, or sleep meds multiplies the risk. Even a single extra tablet can be enough if you’re already taking something that slows your breathing. And don’t assume your grandma’s old Benadryl recipe is safe—her body processes drugs slower now. The same goes for kids. A teaspoon of liquid allergy medicine meant for a 6-year-old can send a toddler to the ER.

If you or someone else takes too much, don’t wait. Call poison control immediately. Don’t try to make them vomit. Don’t give them coffee or anything else "to wake them up." Time matters. Emergency teams use activated charcoal or IV fluids, and sometimes they need to sedate seizures or restart a stopped heart. The good news? Most people recover fully if they get help fast. The bad news? Many wait too long because they don’t realize how dangerous these pills can be.

Below are real stories and expert guides that show you exactly how antihistamines interact with other drugs, which ones carry the highest overdose risk, and how to spot trouble before it’s too late. You’ll find clear comparisons between sedating and non-sedating versions, what to keep out of reach of kids, and how to safely use these meds without putting your life at risk.