When someone takes too much of an antihistamine-whether it’s a child grabbing Benadryl from the cabinet or an adult accidentally doubling their dose-the clock starts ticking. Antihistamine overdose isn’t always obvious at first. The person might seem drowsy, confused, or have a racing heart. But within hours, things can turn dangerous: seizures, irregular heartbeat, trouble breathing, or even coma. The good news? If you act fast, survival rates are over 99%. The key is knowing exactly what to do before emergency help arrives.

What Happens in an Antihistamine Overdose?

Not all antihistamines are the same. First-generation types like diphenhydramine (Benadryl), chlorpheniramine, and promethazine are the real danger. These cross into the brain and block acetylcholine, leading to anticholinergic syndrome. Symptoms include dry mouth, flushed skin, blurred vision, trouble urinating, high heart rate, and hallucinations. At high doses, they also block sodium channels in the heart, which can widen the QRS complex on an EKG-a sign of serious toxicity. A QRS over 100 milliseconds means you’re in cardiac risk territory.

Second-generation antihistamines like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) are much safer. Even large doses usually cause only mild dizziness or a fast heartbeat. But they’re not harmless. A 2020 study of over 9,000 pediatric cases found first-generation antihistamines caused serious outcomes 9 times more often than second-generation ones. Diphenhydramine alone accounted for 83% of severe cases in children under 6.

First Aid: What to Do Immediately

If you suspect an overdose, don’t wait for symptoms. Don’t try to make the person throw up. Don’t give them anything to eat or drink unless instructed. Here’s what actually works:

1. Remove any remaining pills or liquid from the mouth. If it’s a liquid spill on the skin, rinse for 15-20 minutes. If it got in the eyes, flush with cool water for 20 minutes.
2. Call poison control right away. In the U.S., dial 1-800-222-1222. In the UK, call 111 or go to 111.nhs.uk. They’ll ask how much was taken, what kind, when, and the person’s weight and age. Don’t guess-bring the bottle with you.
3. Do not induce vomiting. This is a common myth. If the person is drowsy or confused, vomiting can cause choking or aspiration pneumonia. Poison control will tell you if it’s safe.
4. Keep the person awake and calm. If they’re drowsy, sit them upright. If they’re having trouble breathing, call emergency services immediately.
5. Gather all medication containers. Emergency teams need to know exactly what was taken. Many products combine antihistamines with painkillers or decongestants, which changes the treatment.

One parent on Reddit described finding her 2-year-old with an empty bottle of children’s Benadryl. She called poison control, who calculated the dose at 18 mg-below the 7.5 mg/kg threshold for her child’s weight. They told her to watch for rapid heartbeat, trouble peeing, or extreme sleepiness. She kept the child awake, monitored him for 6 hours, and he was fine. That’s the power of quick, accurate advice.

When to Go to the ER

You don’t need to go to the hospital for every overdose-but you do need to go if:

• The person took more than 7.5 mg/kg of diphenhydramine (for kids under 6) or more than 300 mg total (for adults)
• They have a heart rate over 120 bpm
• They’re confused, hallucinating, or having seizures
• They’re having trouble breathing or their lips are turning blue
• The overdose involved promethazine, especially in infants under 2 (FDA black box warning)

Even if symptoms seem mild at first, cardiac effects can peak 4-6 hours after ingestion. That’s why observation is critical. The standard hospital protocol is 4-6 hours of monitoring for low-risk cases, and 24-48 hours if there’s EKG changes or cardiac instability.

What Happens in the Hospital

Once at the ER, the team will check vital signs, do an EKG, and run blood tests. If the overdose happened within the last 1-2 hours, they’ll give activated charcoal-25-100 grams for adults, 0.5-1 gram per kg for kids. This binds the drug in the gut and stops more from being absorbed.

For cardiac issues, they’ll watch for QRS widening. If it goes over 100 milliseconds, they may give sodium bicarbonate. A 2022 study in the New England Journal of Medicine showed that a 1-2 mEq/kg IV bolus of 8.4% sodium bicarbonate can shorten the QRS interval and stabilize heart rhythm. This isn’t routine-it’s only for clear signs of toxicity.

Agitation or seizures? Benzodiazepines like lorazepam or midazolam are first-line. The American College of Medical Toxicology updated its 2023 guidelines to stop recommending physostigmine-it can cause dangerous heart rhythm problems in overdose patients.

There’s no antidote. Dialysis won’t help. Antihistamines bind tightly to proteins and spread widely through body tissues-so trying to flush them out with fluids or urine doesn’t work. Supportive care is everything: breathing support, IV fluids, heart monitoring, and sedation if needed.

Why Most Overdoses Happen-and How to Prevent Them

The American Association of Poison Control Centers reports that 67% of antihistamine overdoses involve children under 6. Most happen because bottles are left on counters, in purses, or in unsecured medicine cabinets. One in five cases involve dosing errors-parents using kitchen spoons instead of the included cup, or giving two doses thinking the first didn’t work.

Adults aren’t immune. A 24-year-old on Drugs.com shared how he took 400 mg of diphenhydramine trying to sleep, expecting a “stronger high.” He ended up with a heart rate of 130 bpm, blurred vision, and couldn’t urinate for hours. He needed IV fluids and sedatives.

Prevention is simple:

• Store all medications out of reach and sight-preferably locked up.
• Use the measuring device that comes with the bottle. Never use a kitchen spoon.
• Check labels for multiple active ingredients. Many cold medicines contain antihistamines you might not expect.
• Never use first-generation antihistamines to sedate infants or toddlers. The FDA banned this in 2008.
• Know your local poison control number. Save it in your phone.

What’s New in 2025

In June 2023, the FDA required all first-generation antihistamine labels to include clearer overdose warnings and improved child-resistant packaging. The National Poison Data System is also seeing a rise in overdoses from unregulated “natural” allergy remedies that secretly contain diphenhydramine or hydroxyzine. These products often don’t list ingredients, making treatment harder.

But the core treatment hasn’t changed. As Dr. Lewis Nelson from NYU Langone said in 2023: “The fundamental approach hasn’t changed in 20 years because it works.” Activated charcoal, benzodiazepines, sodium bicarbonate for QRS widening, and time-those are the pillars.

Every second counts. Don’t wait. Don’t Google it. Call poison control. Then go to the ER if they say so. Lives depend on quick, correct action.

Next Steps After an Overdose

Once the person is stable, the focus shifts to prevention. If it was accidental, talk to a pharmacist about safer storage and dosing tools. If it was intentional, mental health support is essential. Many adult overdoses are linked to depression or self-harm, and only 18% are suicide attempts-but those still need follow-up care.

Keep the poison control number saved in your phone. Make sure everyone in your household knows where medications are stored. And if you ever have even a small doubt-call. It’s better to be safe than sorry.