December 2, 2022

Symptoms and what to do


Antihistamines are a class of drugs that treat allergy symptoms, such as sneezing, runny nose, and itching. As with any medicine, taking too much can be harmful.

Some antihistamines also work as sleeping pills, as medicine for colds and flu, and as a treatment for motion sickness.

People should follow the dosage directions on the package or their doctor’s advice to avoid overdose. They should also keep antihistamines and all medicines out of the reach of children.

If a person takes too much antihistamine, they can experience unwanted and sometimes serious effects. Always call a doctor or poison control center if you overdose with antihistamines.

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A person who overdoses on antihistamines may have a fever, dilated pupils, and a rapid heart rate.

People can overdose with any type of antihistamine. Antihistamines are a drug, and there is a safe limit to how much a person can take at one time.

This limit depends on several factors, including:

  • the age and height of the person
  • what type of antihistamine they took and how much
  • all the health problems they have
  • other medicine the person is taking

If a healthy adult takes only a slightly higher dose of the antihistamine, such as accidentally taking two tablets instead of one, their symptoms may not be severe or may not have any symptoms.

However, a larger overdose, especially in children or the elderly, can cause severe symptoms. Toxic doses of antihistamines can occur when a person is taking 3 to 5 times the normal amount.

No matter how much a person overdoses, they should see a doctor or call a poison control center.

A position statement of the Canadian Society of Allergy and Clinical Immunology (CSACI) states that first generation antihistamines can have “intolerable and potentially fatal side effects.”

Studies show that first generation antihistamines may affect the central nervous system in high doses.

Examples of first generation antihistamines are:

  • diphenhydramine (Benadryl)
  • chlorpheniramine
  • brompheniramine

These antihistamines can cause nervousness or drowsiness.

Second generation antihistamines usually do not have these effects. Second generation antihistamines are commonly used drugs, including:

  • loratadine (claritin)
  • cetirizine (Zyrtec),
  • fexofenadine (Allegra)

Signs and symptoms of an overdose of antihistamines in children may include:

After these symptoms appear, a child may experience:

  • drowsiness
  • slowed breathing
  • coma

Symptoms of an overdose of antihistamines in adults may include:

  • fever
  • rinsing
  • dilated pupils
  • fast heartbeat (tachycardia)
  • low blood pressure (hypotension)
  • coma
  • an inability to empty the bladder completely (urinary retention)

Second generation antihistamines tend to be less severe with overdose.However, any overdose can be dangerous and people should seek help from a healthcare practitioner or poison control center immediately.

If a person suspects an antihistamine overdose, they should call the American Association of Poison Control Centers (AAPCC) at 1-800-222-1222. The AAPCC hotline is open 24 hours a day and is free.

If a person has severe symptoms, such as difficulty breathing, a fast or slow heartbeat, seizures, or loss of consciousness, call 911 or go to the nearest emergency room.

Some antihistamines are safe for children. However, the Food and Drug Administration (FDA) Be careful, just because a product says it is for children does not mean that a child of any age can take it.

Antihistamines Research says children and the elderly are at greater risk of overdosing on antihistamines than adults.

A study discovered that the children had heart problems, seizures and even death from overdosing on antihistamines. The most common reactions, however, are mild and include:

Second generation antihistamines are less likely to cause toxicity in children than first generation antihistamines, according to a study over 9,000 children.

However, another study found that second generation antihistamines can cause dangerous symptoms when children overdose.

The diagnosis of an antihistamine overdose usually begins with a discussion of all the medications the person is taking, not just the antihistamines. This is because some other medicines, such as cold, flu and sleep medicines, contain antihistamines.

If a person takes any of these other medicines and also takes an allergy medicine, they can accidentally overdose.

Other drugs, like some Antidepressants and medicine for motion sickness can also cause an overdose if a person mixes them with antihistamines.

If someone is not sure how much antihistamine they have taken, a doctor may need to do a physical exam. They may look for signs such as drowsiness, irritability, blurred vision, or seizures. They can also check heart rate, respiratory rate, and blood pressure.

In some cases, a person may need blood tests or further observation in a hospital.

In mild cases, a person may be able to use their own care at home. However, they should still call the AAPCC or a doctor to make sure this is safe.

If the person has severe symptoms, they may need hospitalization, including heart monitoring, intravenous (IV) fluids, and medications.

In some cases, doctors may give a person activated charcoal, which helps prevent the absorption of certain drugs and chemicals in the gut. They can also use ipecac syrup, which makes the person vomit. This can cause any excess medicine to leave the body in vomit.

Antihistamines can be first generation (sedating) or second generation (non-sedating). These two types can cause different reactions if a person takes too much.

First generation antihistamines are more likely to cross the blood brain barrier. This means that they can make people dizzy or drowsy, even in normal doses.

Second generation antihistamines are newer drugs and are less likely to have these effects. Nonetheless, people can overdose on both types.

First generation over-the-counter antihistamines include:

  • diphenhydramine (Benadryl)
  • chlorpheniramine (Chlor-Trimeton)
  • brompheniramine (Dimetapp)
  • clemastine (Tavist)
  • doxylamine (Unisom)
  • hydroxyzine (Vistaril)
  • promethazine (Promethégane)
  • triprolidine (Actidil)

These drugs can act as sedatives, so they can make a person feel tired or dizzy.

Some experts say first generation antihistamines are more likely to be fatal by accident or overdose than their second generation counterparts.

Second and third generation antihistamines available over the counter include:

  • cetirizine (Zyrtec)
  • fexofenadine (Allegra)
  • loratadine (claritin)

These newer antihistamines are less likely to cause drowsiness and dizziness. Search found them less likely to cause dangerous effects if a person takes too much.

Antihistamines are generally safe when a person takes the right amount. However, as with any medicine, they can cause serious side effects if someone takes too much.

People should keep antihistamines and all medicines out of the reach of children. Some antihistamines are safe for children, but it is important to ask a doctor or pharmacist for the correct dose.

Different antihistamines have different strengths. It is essential to read labels carefully to make sure a person is not taking too much.

In case of overdose, seek medical advice immediately or call AAPCC at 800-222-1222.