Emergency Treatment for Antihistamine Overdose: First Steps You Must Take

When a child swallows a handful of Benadryl pills, or an adult accidentally takes four times their dose of cetirizine, the clock starts ticking. Antihistamine overdose isn’t always obvious-no screaming, no blood, just drowsiness, a racing heart, or dry skin. But in the first few hours, what you do can mean the difference between a quick recovery and a trip to the ICU. The good news? With the right steps, survival rates are nearly 100%. The bad news? Most people wait too long to act.

What Happens in an Antihistamine Overdose?

Not all antihistamines are the same. First-generation ones like diphenhydramine (Benadryl), chlorpheniramine, and promethazine are the real danger. They cross into the brain and block acetylcholine, causing what’s called anticholinergic toxicity. Symptoms include confusion, flushed skin, dilated pupils, trouble urinating, and a heartbeat that races past 120 beats per minute. At high doses, they also block sodium channels in the heart-this is what causes the QRS complex on an EKG to widen beyond 100 milliseconds. When it hits 120 milliseconds or more, you’re looking at a risk of seizures, ventricular arrhythmias, or even cardiac arrest.

Second-generation antihistamines like loratadine, cetirizine, and fexofenadine are much safer. They’re designed not to cross the blood-brain barrier. Even if someone takes 10 times the normal dose, they’re more likely to feel dizzy or have a mild fast heart rate than end up in critical condition. But don’t assume they’re harmless. A 2020 study of over 9,000 pediatric cases found that while only 0.2% of second-gen overdoses led to serious outcomes, 1.8% of first-gen cases did. And diphenhydramine alone was behind 83% of those serious cases in kids under six.

First Steps: What to Do Right Now

If you suspect someone has taken too much antihistamine, don’t wait for symptoms. Don’t try to make them throw up. Don’t give them coffee or water to "flush it out." Here’s what actually works:

1. Remove any remaining pills or liquid from the mouth. Don’t induce vomiting-this increases the risk of choking, especially if the person is drowsy or unconscious.
2. Call poison control immediately. In the U.S., dial 1-800-222-1222. In Australia, call 13 11 26 (Poisons Information Centre). Don’t search online. Don’t wait. Poison control centers have real-time databases that tell them exactly how dangerous the dose is based on weight, age, and the specific drug. They’ve seen thousands of cases. They know when to send someone to the hospital and when it’s safe to watch at home.
3. Gather the medication container. Emergency responders need to know the exact name and strength. A bottle labeled "Benadryl" could mean 25 mg diphenhydramine per tablet-or it could be a combination product with pseudoephedrine. That changes everything.
4. If the substance got on the skin or in the eyes, rinse immediately. For skin contact, wash under running water for 15-20 minutes. For eye exposure, flush with cool water for at least 20 minutes. Don’t rub.

When to Go to the ER

You don’t need to rush to the hospital for every accidental sip. The Victorian Poisons Information Centre guidelines say that for children under six, an ingestion under 7.5 mg per kilogram of body weight (e.g., under 37.5 mg for a 5 kg child) is usually safe to monitor at home. For adults, less than 300 mg of diphenhydramine is often manageable without hospitalization.

But if any of these are true, go now:

• Heart rate over 120 bpm
• QRS widening on EKG (greater than 100 ms)
• Seizures, tremors, or hallucinations
• Difficulty breathing or unresponsiveness
• Ingestion of more than 300 mg in an adult or 7.5 mg/kg in a child

What Happens in the Hospital

Once at the ER, the focus is on stabilizing the airway, breathing, and circulation. The first thing they’ll do is check the EKG. If the QRS is widened, they’ll start thinking about sodium bicarbonate. That’s right-baking soda. 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 reverse QRS prolongation caused by diphenhydramine. It works by alkalizing the blood and improving sodium channel function.

Activated charcoal is given within 1-2 hours of ingestion, if the patient is awake and able to swallow. For adults, that’s 25-100 grams. For kids, it’s 0.5-1 gram per kilogram. It binds the drug in the gut and stops more from being absorbed. After that, it’s mostly about watching and waiting.

Benzodiazepines like lorazepam or midazolam are used if the person is agitated, having seizures, or showing signs of extreme delirium. Don’t use physostigmine. That was once a standard treatment, but the American College of Medical Toxicology updated its guidelines in 2023 to warn against it. It can cause dangerous heart rhythms in antihistamine overdose.

Most patients who don’t have cardiac issues are observed for 4-6 hours. If everything stays stable, they can go home. Those with EKG changes or severe symptoms are kept for 24-48 hours for continuous monitoring. The median hospital stay for admitted cases is just over 22 hours.

Why People Get It Wrong

Most overdoses aren’t intentional. In fact, 79% are accidents. Parents mix up dosing cups. Kids grab bottles left on nightstands. Teens take "sleep aids" to get high. One Reddit user described their 2-year-old swallowing five 25 mg tablets of diphenhydramine-125 mg total. They called poison control, who said it was under the danger threshold for their child’s weight. They watched for six hours. The child slept more than usual but woke up fine.

But another user, a 24-year-old who took 400 mg thinking it would help with anxiety, ended up in the ER with a heart rate of 130 bpm, dry mouth so bad they couldn’t swallow, and urinary retention. They spent 36 hours in the hospital. "I thought I was just being extra careful," they wrote. "I didn’t know one pill could be dangerous if you took too many."

Prevention Is the Best Treatment

The FDA issued a black box warning in 2006 against using sedating antihistamines like promethazine in infants under two. The European Medicines Agency banned their use for insomnia in children. But many over-the-counter products still contain them-cold medicines, sleep aids, allergy blends. Always check the active ingredients. If it says "diphenhydramine," "chlorpheniramine," or "promethazine," treat it like a controlled drug.

Store all medications out of reach. Use child-resistant caps-even if you think you’ll remember. Use a dosing syringe, not a cup. Write down the time you give the dose. A 2022 AAPCC report found that 42% of pediatric antihistamine exposures come from dosing errors.

What You Should Know

- Antihistamine overdose is rarely fatal if treated quickly. Survival rate: 99.9% with proper care.

- First-generation antihistamines are 9 times more likely to cause serious harm than second-generation ones.

- Sodium bicarbonate is a proven, life-saving treatment for QRS widening.

- Never induce vomiting. It’s dangerous and ineffective.

- Poison control is your best first step-not Google, not a friend, not the ER waiting room.

Final Advice

If you’re ever unsure whether someone overdosed on antihistamines, call poison control. Don’t guess. Don’t wait. Don’t rely on internet advice. The system is built to handle these calls-24/7, for free. In 2022, U.S. poison centers handled over 2 million exposure cases. Antihistamines made up 4.3% of them. You’re not alone. And with the right first steps, you can prevent a tragedy before it starts.