What Happens When You Overdose on Hydroxyzine?

A hydroxyzine overdose typically causes heavy sedation, a fast heartbeat, and dry mouth, with most cases resolving without lasting harm. In a large analysis of over 17,000 hydroxyzine poisoning cases reported to the U.S. National Poison Data System, mortality was rare, occurring in roughly 0.02% of reported exposures.¹ That said, large overdoses can produce seizures, dangerous heart rhythms, and respiratory depression, and the drug’s effects on the heart have drawn increasing scrutiny from regulators and researchers in recent years.

The Most Common Symptoms of Overdose

Hydroxyzine is a first-generation antihistamine that also blocks certain receptors in the nervous system responsible for wakefulness and muscle coordination. When someone takes far more than a prescribed dose, the most immediate and noticeable effect is profound drowsiness, often progressing to a stupor. In the large poisoning-registry analysis mentioned above, mild central nervous system depression was actually more common in hydroxyzine-poisoned patients than in those who overdosed on diphenhydramine (the active ingredient in Benadryl), even though more serious outcomes were less frequent.¹

Beyond sedation, overdose features commonly include a rapid heart rate (sinus tachycardia), dry mouth, poor coordination, agitation, and occasionally psychosis. Convulsions can occur but are uncommon. A case report of a 13-month-old child who accidentally ingested a large quantity of hydroxyzine documented the main signs of toxicity as generalized seizures and sinus tachycardia, with a blood hydroxyzine level far above the therapeutic range.² Blood pressure can swing in either direction, and some patients develop low blood pressure that requires intravenous fluids.

Anticholinergic Effects and Why They Matter

Much of what makes an antihistamine overdose unpleasant comes from a side pathway: the drug also blocks acetylcholine receptors, which are involved in everything from sweating to digestion to pupil size. At therapeutic doses, hydroxyzine has relatively low affinity for these muscarinic receptors compared to some other first-generation antihistamines.³ But in overdose, when blood levels soar, even a weak receptor-blocker starts producing noticeable anticholinergic symptoms.

These include dilated pupils, flushed and dry skin, urinary retention, and a characteristic inability to sweat despite having a rising body temperature. Clinicians sometimes use the mnemonic “blind as a bat, dry as a bone, red as a beet, mad as a hatter, hot as a hare” to remember the full anticholinergic picture. The research comparing hydroxyzine and diphenhydramine overdoses found that hydroxyzine-poisoned patients consistently had lower rates of developing classic anticholinergic findings and were less likely to receive physostigmine (the antidote for anticholinergic toxicity), with hyperthermia being a notable exception in one dataset.¹ In practical terms, a hydroxyzine overdose tends to look “less anticholinergic” than an equivalent diphenhydramine overdose, but the symptoms still appear in a significant number of cases.

The Heart Rhythm Concern

The most worrying aspect of hydroxyzine overdose is its effect on the heart’s electrical system. Hydroxyzine blocks a specific potassium channel in the heart (called hERG) in a dose-dependent way, and this blockade can lengthen the interval between heartbeats in a way that shows up on an electrocardiogram as “QT prolongation.”⁴ When the QT interval gets long enough, the heart becomes vulnerable to a chaotic, potentially fatal arrhythmia called torsade de pointes.

Lab studies have shown that hydroxyzine also inhibits sodium and calcium channels in the heart, which could partially offset the QT-prolonging effect of hERG blockade.⁴ This partial compensation may help explain why fatal arrhythmias from hydroxyzine are not as common as you might expect given the hERG findings alone. Still, cases of torsade de pointes from hydroxyzine do occur. One published case involved a 43-year-old man who developed the arrhythmia while on hydroxyzine and was treated successfully with magnesium sulfate and isoproterenol after the drug was stopped.⁵ Another case report described a patient whose torsade de pointes proved difficult to manage even with multiple antiarrhythmic drugs.⁶

The cardiac risk is especially relevant for people who already have a prolonged QT interval (whether inherited or caused by another medication), those with low potassium or magnesium levels, and those taking other QT-prolonging drugs at the same time. European regulators issued a warning about hydroxyzine and QT prolongation in 2015, leading to dose-cap recommendations in some countries. In an overdose situation, the heart rhythm risk is amplified considerably because blood levels of the drug far exceed anything seen with normal dosing.

How Hydroxyzine Is Processed and Why That Matters in Overdose

Hydroxyzine is rapidly broken down by the liver into several metabolites, the most important being cetirizine, which is itself a widely used antihistamine (sold over the counter as Zyrtec).⁷ Cetirizine crosses into the brain much less readily than hydroxyzine, which is why it causes far less drowsiness at normal doses. A clinical trial comparing the two drugs at therapeutic doses found that hydroxyzine caused significantly more drowsiness than cetirizine, which performed no differently from placebo on that measure.⁸

This metabolism matters in overdose because anything that slows the liver’s ability to break hydroxyzine down will keep blood levels higher for longer. For example, cimetidine (an older heartburn drug) has been shown to significantly increase hydroxyzine’s blood concentration by interfering with the same liver enzymes.⁹ Liver disease, other enzyme-inhibiting drugs, and even grapefruit juice could theoretically raise hydroxyzine levels and worsen the effects of an overdose, though large-scale data on these interactions in overdose settings are sparse.

Children and Older Adults Face Greater Risk

Children are disproportionately represented in accidental hydroxyzine poisoning reports, partly because the drug is frequently prescribed as a liquid and partly because pediatric doses are weight-based and easy to miscalculate. The case of the 13-month-old mentioned earlier illustrates how quickly a young child can progress to seizures after a large accidental ingestion.² Children have immature liver metabolism and smaller body mass, so a given milligram amount produces much higher blood concentrations than it would in an adult.

Older adults face a different set of vulnerabilities. Aging naturally reduces kidney function, liver enzyme activity, and the body’s water-to-fat ratio, all of which slow drug clearance and raise effective drug levels. On top of that, older people are far more sensitive to anticholinergic effects, even at normal doses. The spectrum of problems ranges from sedation and mild confusion all the way to frank delirium, and these effects layer on top of whatever cognitive baseline the person already has.¹⁰ An older adult who takes a modest overdose may experience confusion and agitation severe enough to mimic a stroke or a sudden worsening of dementia, and the connection to hydroxyzine is sometimes missed in the emergency department if the medication history is not immediately available.

What Happens in the Emergency Department

There is no specific antidote that reverses all of hydroxyzine’s effects the way naloxone reverses an opioid overdose. Treatment is primarily supportive: monitoring heart rhythm, maintaining blood pressure, protecting the airway if the person is too sedated to breathe safely, and controlling seizures if they occur.

For anticholinergic symptoms specifically, physostigmine is sometimes used as a diagnostic and therapeutic tool. A slow intravenous dose can temporarily reverse anticholinergic toxicity, and in the pediatric case report, seizure control with physostigmine plus general supportive care led to complete recovery within 72 hours.¹¹ Physostigmine is not used casually, though, because it can slow the heart and trigger its own seizures if given too fast or to the wrong patient.

When the heart rhythm is the main concern, treatment may include intravenous magnesium sulfate and, in refractory torsade de pointes, electrical pacing or isoproterenol to speed the heart rate and shorten the QT interval.⁵ Rare but documented complications of severe hydroxyzine overdose include distributive shock (a dangerous drop in blood pressure caused by widespread blood vessel relaxation) and acute respiratory distress syndrome, both of which require intensive care management.¹²

Activated charcoal may be considered if someone presents within an hour or two of ingestion, but it is not always given, particularly if the person is already drowsy and at risk of vomiting and aspirating. Gastric lavage (stomach pumping) is rarely performed anymore for antihistamine overdoses. In most cases, the strategy is to keep the patient safe, treat complications as they arise, and wait for the drug to be metabolized out of the system.

How Hydroxyzine Compares to Other Antihistamines in Overdose

Among first-generation antihistamines, hydroxyzine’s overdose profile sits somewhere in the middle of the severity spectrum. The large poisoning-registry comparison found that hydroxyzine-poisoned patients were less likely than diphenhydramine-poisoned patients to develop coma, respiratory depression, seizures, or ventricular arrhythmias, and were less likely to need intubation.¹ Diphenhydramine has stronger anticholinergic binding and also blocks sodium channels in the heart more aggressively, which may explain why it causes more severe toxicity at high doses.

That relative mildness should not be confused with safety. Hydroxyzine’s hERG channel blockade gives it a cardiac risk profile that diphenhydramine does not share in quite the same way, and the fact that torsade de pointes cases have been documented even at therapeutic doses in vulnerable patients makes it a drug that deserves respect.⁶ Second-generation antihistamines like cetirizine and loratadine, which were specifically designed to stay out of the brain, are far less toxic in overdose. Cetirizine, as hydroxyzine’s own metabolite, provides equivalent itch relief with a much wider safety margin.⁸

Mortality and the Role of Co-Ingestion

Death from hydroxyzine alone is uncommon. The two datasets in the large registry study reported mortality rates of 0.02% and 0.8%, respectively, among hydroxyzine-poisoned patients.¹ The higher figure comes from a registry of cases managed by toxicology consultants, which naturally captures sicker patients. Still, even the more alarming number reflects a drug that is rarely lethal on its own.

Most fatal outcomes attributed to hydroxyzine in the forensic literature involve co-ingestion of other central nervous system depressants such as opioids, benzodiazepines, or alcohol. These combinations amplify each drug’s sedative and respiratory-depressant effects, and the interaction is more than simply additive. A forensic study examining postmortem hydroxyzine concentrations found wide variability in blood levels, with peripheral blood concentrations ranging from 0.07 to 3.0 mg/L across cases. The study also noted that hydroxyzine is prone to moderate postmortem redistribution from tissues into blood, which can artificially inflate blood levels measured after death and complicate the interpretation of cause-of-death findings.¹³

Postmortem Redistribution and Forensic Interpretation

When someone dies and a toxicology screen shows hydroxyzine, the numbers are not straightforward to interpret. After death, drugs stored in tissues like the liver can leak back into the blood, raising measured blood concentrations above what they actually were at the time of death. Forensic scientists gauge this tendency by comparing liver-to-blood ratios. For hydroxyzine, the average liver-to-peripheral-blood ratio was about 14, which falls in the range that suggests a moderate degree of redistribution after death.¹³

This matters because a coroner looking at a postmortem blood level of, say, 2.5 mg/L needs to know whether that number reflects what was circulating when the person was alive or whether it was inflated by tissue leakage after death. Peripheral blood (drawn from the femoral vein, in the leg) is generally more reliable than central blood (drawn near the heart) for this reason, and the study confirmed that central-to-peripheral blood ratios for hydroxyzine averaged close to 1, meaning the two sites were fairly concordant. The practical takeaway for death investigations is that hydroxyzine blood levels should be interpreted cautiously and in context, particularly in mixed-drug cases where the contribution of each substance needs to be weighed carefully.

When to Seek Medical Attention

If you or someone you know has taken more hydroxyzine than prescribed, the threshold for seeking medical help should be low. Even though most cases resolve without permanent harm, the cardiac risk makes it impossible to predict from symptoms alone whether an overdose will stay mild. A person who seems only drowsy can develop a dangerous heart rhythm without warning, particularly if they take other medications that also affect the QT interval, have an electrolyte imbalance, or have underlying heart disease.

Poison control centers (in the United States, reachable at 1-800-222-1222) can help triage the situation over the phone and advise whether emergency department evaluation is needed. For children who have gotten into a parent’s medication, the answer is almost always yes. For intentional overdoses of any size, emergency evaluation is always appropriate, both for the medical risk and because the circumstances demand a safety assessment. The reassuring statistic is that with prompt supportive care, full recovery within a few days is the expected outcome for the vast majority of hydroxyzine overdoses.¹¹