Antihistamine Types & Side Effects

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Antihistamines are common over-the-counter and prescription medications that help with allergy symptoms. They can also help manage other conditions, like digestive issues and motion sickness. Because there are so many options to choose from, it’s best to talk to your healthcare provider or pharmacist about the right one for you.

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Antihistamines are a class of medication that commonly treat allergy symptoms, like runny nose, sneezing and itchy eyes. They can also treat some digestive conditions.

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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Most antihistamines are over-the-counter (OTC) medications, but they also come in prescription form. Antihistamines are also common ingredients in the following OTC medications:

• Cold and flu products
• Decongestants
• Cough suppressants
• Pain relievers

Antihistamines work by blocking histamine receptors. “Anti-” means “against.” Histamine is a chemical your immune system releases to send messages between different cells. Histamine has several functions, but it’s mainly known for its role in causing allergic and anaphylactic symptoms. Allergic reactions can happen due to several substances (allergens), including:

• Environmental allergens, like pollen, dust, mold and pet dander
• Insect bites and stings
• Foods
• Medications

Antihistamines can help manage all these allergies.

Healthcare providers divide antihistamines into two major subtypes based on which histamine receptors they block:

• H1 receptors.
• H2 receptors.

You have H1 receptors throughout your body, including in neurons (brain cells), smooth muscle cells of your airways and blood vessels. Activation of the H1 receptors causes the well-known allergy and anaphylaxis symptoms. So, H1 receptor antihistamines mainly treat allergy symptoms.

The U.S. Food and Drug Administration (FDA) has approved the use of H1 antihistamines to help treat the following conditions:

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• Allergic rhinitis (hay fever)
• Allergic conjunctivitis (pink eye)
• Allergic skin reactions, like atopic dermatitis (a condition that causes skin rashes and itchiness)
• Sinus infection (sinusitis)
• Hives
• Angioedema (skin swelling that often happens from an allergic reaction)
• Bronchitis
• Motion sickness
• Nausea
• Vomiting.

Healthcare providers may also use H1 antihistamines to treat insomnia. This is an off-label use of the medication.

The H1 antihistamines further break down into two groups:

• First-generation antihistamines.
• Second-generation antihistamines.

First-generation antihistamines generally have more side effects than second-generation antihistamines because of how they work. First-generation antihistamines more easily cross your blood-brain barrier into your central nervous system. This causes drowsiness. Second-generation antihistamines are generally safer because they don’t cause drowsiness and interact with fewer medications.

A few examples of first-generation H1 antihistamines include:

• Brompheniramine chewable tablets (BroveX CT®)
• Chlorpheniramine tablets (like Ahist®,Aller-Chlor® and Chlor-Trimeton®)
• Clemastine tablets (Dayhist®, Tavist Allergy®)
• Cyproheptadine oral liquid (Periactin®)
• Dimenhydrinate chewable tablets (Dramamine®)
• Diphenhydramine capsules or tablets (like Alka-Seltzer Plus Allergy®, Benadryl® and Vicks ZzzQuil Nighttime Sleep-Aid®)
• Doxylamine tablets (Unisom®)
• Hydroxyzine capsules or tablets (Atarax®, Rezine® and Vistaril®)

A few examples of non-drowsy H1 antihistamines include:

• Azelastine eye solution (Optivar®)
• Azelastine nasal spray (Astelin®, Astepro®)
• Cetirizine tablets (Zyrtec®)
• Desloratadine (Clarinex®)
• Fexofenadine (Allegra®)
• Loratadine capsules or tablets (Claritin®)

You have H2 receptors mainly in the cells in your stomach that release acid, smooth muscle cells and heart cells. So, H2 antihistamines treat certain gastrointestinal (GI) conditions.

The FDA approves H2 antihistamines to help treat the following conditions:

Healthcare providers may also use H2 antihistamines to treat indigestion. This is an off-label use of the medication.

If you’re taking an antihistamine to help with stomach issues, you’re likely taking an H2 antihistamine. A few examples include:

• Cimetidine (Acid Reducer®, Tagamet®)
• Famotidine (Pepcid®)
• Nizatidine (Axid®)

Antihistamines work by blocking or reducing a chemical called histamine. It has a role in several different bodily processes. Histamine:

• Widens blood vessels (vasodilation)
• Helps fluids move through blood vessel walls
• Plays a role in inflammation
• Affects muscle contractions in your intestines and lungs
• Stimulates stomach (gastric) acid secretion
• Affects your heart rate
• Helps transmit messages between nerve cells

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Your body releases histamine when it encounters a threat from an allergen, leading to allergy symptoms, like:

• Nasal congestion or runny nose
• Sneezing
• Hives and other skin rashes
• Itchy or watery eyes

By blocking the effects of histamine, antihistamines help prevent or manage these symptoms. Histamine also affects your nervous system and digestive system. So, antihistamines can also help with certain digestive conditions and things like motion sickness and nausea.

Antihistamines don’t always completely control symptoms. This is because other chemicals besides histamine may be partly responsible for the symptoms. Antihistamines only block histamine.

Antihistamines come in several forms, including:

• Tablets and capsules you swallow
• Chewable tablets
• Syrups
• Lotions, creams and gels for your skin
• Eye drops
• Nasal sprays
• Suppositories (medication inserted into your rectum)
• Injections (these are mainly for hospital use)

There are many types of antihistamines, and each one is different. Make sure to carefully read the label and information for directions that come with the medication. Or follow your pharmacist’s or healthcare provider’s instructions.

Before taking an antihistamine, you should know:

• How to take it: You may need to take the medication with food or water. If it’s in the form of eye drops, cream or a nasal spray, you’ll need to know how to apply it correctly.
• How much to take (the dose): Proper doses vary depending on the type of medication, your age and your weight. The number of pills or eye drops, for example, varies, as well.
• When and how often you can take it: The packaging will have information on what time of day and how often you can take it. For example, you need to take some types of antihistamines before you go to bed. Some antihistamines you only take once a day. Others, you can take more than once a day.
• How long you can take it: You can take some types of antihistamines long term. But others are only for short-term use (a few days).

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In terms of what not to do when taking antihistamines:

• Do NOT “double-up” or increase a dose.
• Do NOT take a dose sooner than you’re supposed to.
• Do NOT take two different antihistamines at the same time (unless your healthcare provider tells you to).
• Do NOT give over-the-counter cough and cold antihistamines to children under 4.
• Do NOT drive a vehicle or operate heavy machinery while taking a first-generation antihistamine. They can cause sleepiness and slow reaction times, which can be dangerous.

Children and people over the age of 65 are more sensitive to the effects of antihistamines. Use caution and talk to a healthcare provider first.

People take antihistamines for several reasons. When you take an antihistamine likely depends on what kind of allergies you have and how severe they are. For example, you may take an antihistamine:

• Every day to help keep daily symptoms under control or prevent them. People who have a pet allergy and own a pet may do this, for example.
• Only when you have symptoms (like if you have an allergic reaction to an insect bite).
• Before you’re exposed to things that often cause your allergy symptoms (like if you have a cat allergy and you’re visiting a friend who has a cat).

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For many people with environmental allergies, their symptoms are the worst in the early morning. Taking an antihistamine at bedtime may help you or your child feel better in the morning.

Again, if you don’t know what’s best for you or have questions, talking to a healthcare provider or pharmacist is always your best bet.

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Each antihistamine has different possible side effects. You should discuss taking an antihistamine with your healthcare provider. Together, you can decide if the potential benefits of an antihistamine outweigh its potential side effects.

Side effects of first-generation H1 (allergy) antihistamines can include:

• Drowsiness (sleepiness) and reduced coordination, reaction speed and judgment
• Dry mouth and dry eyes
• Blurred or double vision
• Dizziness
• Headaches
• Difficulty peeing and constipation
• Low blood pressure
• Mucous thickening in your airways

Side effects of second-generation H1 (allergy) antihistamines can include:

• Headaches
• Cough
• Sore throat
• Abdominal pain or discomfort
• Nausea or vomiting
• Drowsiness (in high doses)

Side effects of H2 (digestive) antihistamines are uncommon but can include:

• Fatigue
• Diarrhea or constipation
• Joint or muscle pain
• Headache
• Confusion in people over the age of 65
• Dizziness
• Enlarged male breast tissue (gynecomastia)
• Milky nipple discharge (galactorrhea)

Antihistamines help millions of people with allergies manage their symptoms. And they’re one of the most commonly used medications. Antihistamines are generally effective, though it may take time to find the one that works best for you.

Even though many antihistamines are available over the counter, they can still cause significant problems if you don’t use them correctly.

The main possible complication is overdose (antihistamine toxicity).

In addition, some antihistamines can make certain health conditions worse. If you have any of the following conditions, you should talk to your healthcare provider first before taking antihistamines:

• Diabetes
• Enlarged prostate or problems passing pee (urinary retention)
• Epilepsy
• Heart disease
• High blood pressure
• Increased pressure in your eye (glaucoma)
• Hyperthyroidism (overactive thyroid)

The toxicity of antihistamines varies greatly depending on the type. But first-generation H1 antihistamines (the ones that make you drowsy) are most likely to lead to antihistamine overdoses.

An overdose can happen if you or your child take too much of an antihistamine. Or it can happen if you take it with other types of medications, like hypnotics or sedatives. These are most often sleeping pills. Children and people over the age of 65 are most at risk for accidental antihistamine overdose.

Symptoms of antihistamine overdose include:

• Intense drowsiness
• Hallucinations
• Agitation and irritability (more common in children than adults)
• Blurred or double vision
• Fast heart rate (tachycardia)
• Seizures

If you or your child has these symptoms, call 911 (or your local emergency number) or get to the emergency room as soon as possible. A severe antihistamine overdose can be fatal if it’s not treated.

To prevent overdoses, always take your medication as prescribed and store antihistamines safely away from children and pets.

It’s safest to talk to your healthcare provider if you’re pregnant, planning to become pregnant or are breastfeeding.

Up to 15% of people use an antihistamine during pregnancy to manage allergic rhinitis or nausea. Many studies show no significant risk of fetal malformations (birth defects) with first-generation antihistamines. Small amounts of antihistamines can pass on to your baby if you breastfeed.

Everyone is different, and researchers are always learning new information. Because of this, your provider will want to talk with you and help you decide what’s best.

Even though you can get most antihistamines at the store, it’s still a good idea to talk to your healthcare provider about taking them. This is especially true if you’re thinking about giving an antihistamine to your child or if you have other health conditions.

Otherwise, call your provider if:

• You’re having nosebleeds, or you have any other new nasal symptoms.
• You’re having bothersome side effects.
• Your allergy symptoms aren’t improving.
• You’re having trouble taking your antihistamine medication.
• You’re thinking of switching antihistamines.
• You’re thinking of stopping the medication (if it’s a prescription).

It might help to ask your provider the following questions:

• What type of antihistamine would work best for me?
• How do I properly take antihistamines?
• What side effects should I look out for?
• What antihistamines won’t interfere with the current medications I take?
• Can I live my life normally while using this medication? Can I drive? Can I operate heavy machinery?
• Can I take antihistamines if I’m pregnant or breastfeeding?
• Are antihistamines safe for my child?
• What will happen if I don’t take an antihistamine to help with my allergies?

Histamine tries its best to help and protect you. But it can be oversensitive and overreact, leading to symptoms like rashes, runny nose and watery eyes. That’s where antihistamines can help. If you have allergies, stomach issues or any other conditions and symptoms mentioned in this article, talk to your healthcare provider about your antihistamine options.

Always check with your provider or pharmacist if you have concerns about antihistamines, and always follow the directions on the labels.

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Cleveland Clinic’s health articles are based on evidence-backed information and review by medical professionals to ensure accuracy, reliability and up-to-date clinical standards.

Cleveland Clinic’s health articles are based on evidence-backed information and review by medical professionals to ensure accuracy, reliability and up-to-date clinical standards.

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