Antihistamines

Antihistamines are one of the most commonly used classes of medications for managing allergy symptoms. They are widely available in both prescription and over-the-counter (OTC) forms and are used across many healthcare settings. Antihistamines help relieve symptoms caused by allergic reactions, such as sneezing, a runny nose, watery eyes, and itching.

These medications have been used for decades and are considered a cornerstone of allergy medications. Antihistamines are commonly used for seasonal allergies, hay fever, hives, and other allergic reactions. Some are also used for motion sickness, nausea, sleep support, and anxiety-related symptoms.

Antihistamines come as pills, chewable tablets, capsules, liquids, eye drops, and nasal sprays. There are also injectable formulations used mainly in health care settings.

Antihistamines are medications that block the effects of histamine, a chemical released by the immune system during an allergic reaction. Histamine plays a key role in allergy symptoms by binding to the H1 receptor on cells in the nose, eyes, skin, and airways.

Antihistamines work by blocking histamine receptors (H1 receptors) on cells, which stops histamine from attaching and triggering allergy symptoms. Histamine is released when the body reacts to an allergen such as pollen, dust mites, pet dander, or certain foods.

By blocking the effects of histamine, antihistamines reduce swelling, mucus production, and itching. This explains how antihistamines work to relieve sneezing, a runny nose, watery eyes, and skin reactions. Some antihistamines also affect the central nervous system, which can lead to drowsiness.

In the stomach, they also stimulate stomach (gastric) acid secretion and affect stomach muscle contractions.

Antihistamines are divided into first-generation antihistamines and second-generation antihistamines. The main difference between these groups is how likely they are to cause sleepiness and other side effects.

First-generation antihistamines readily cross into the brain and often cause sedation. They are effective but more likely to cause drowsiness and anticholinergic side effects.

Diphenhydramine (Benadryl).

Chlorpheniramine (Chlor-Trimeton).

Brompheniramine (Dimetapp, Bromfed, Lodrane).

Clemastine (Tavist).

Hydroxyzine (Vistaril, Resine, Atarax).

Doxylamine (Unisom).

Dimenhydrinate (Dramamine).

Second-generation non-sedating antihistamines are newer and cause less drowsiness. They are often preferred for long-term allergy management, and they are also used for digestive issues.

Cetirizine (Zyrtec).

Cimetidine (Tagamet).

Famotidine (Pepcid).

Nizatidine (Axid).

Levocetirizine (Xyzal).

Loratadine (Claritin).

Desloratadine (Clarinex).

Fexofenadine (Allegra).

Some antihistamines are available as nasal sprays or eye drops, such as Azelastine (Astepro) nasal spray.

Antihistamines are used to treat a wide range of allergy-related conditions.

Allergic rhinitis and hay fever: to relieve symptoms of allergic rhinitis and hay fever, including sneezing, runny nose, watery eyes, and nasal itching.

Hives (urticaria) and skin reactions: for treating itching caused by allergic reactions to foods, medications, or insect stings.

Motion sickness and nausea: certain first-generation antihistamines help prevent motion sickness and dizziness by affecting the inner ear and central nervous system.

Digestive conditions: to relieve symptoms of peptic ulcer disease, GERD, gastritis, or Zollinger-Ellison syndrome.

Some antihistamines are used for anxiety, sleep support, or as part of treatment for severe allergic reactions under medical supervision.

Side effects of antihistamines vary depending on the generation and specific medication.

Common side effects include:

Drowsiness, especially with first-generation antihistamines.

Dry mouth or dry eyes.

Dizziness.

Constipation or urinary retention.

Second-generation antihistamines generally cause fewer side effects, but some people may still experience mild drowsiness. The side effects of antihistamines are more pronounced when combined with alcohol or other sedating medications.

These are not all of the possible side effects of antihistamines. You should always seek medical advice from your healthcare provider for any questions or concerns about your medical condition or treatment. Read all patient information, medication guides, or drug information sheets that come with this medication. You can also report adverse effects to the Food and Drug Administration at www.fda.gov/medwatch or 1-800-FDA-1088.

Although these medications are generally well-tolerated and safe, there are certain warnings, precautions, and drug interactions you should be aware of.

Warnings

Before starting antihistamines, it is crucial to discuss any existing health conditions with your healthcare provider. You should be sure that your healthcare provider is aware of all your medical conditions, including:

Diabetes.

Enlarged prostate.

Epilepsy.

Heart disease.

High blood pressure.

Glaucoma.

Hyperthyroidism (overactive thyroid).

Boxed Warning

Promethiazine has a boxed warning for use in children under 2 due to risks of fatal respiratory depression, and warnings for older children.

First-generation antihistamines have warnings for sedation and cognitive impairment, especially in the elderly. They also carry a risk of QT prolongation and the risk of seizures, coma, or death from high doses.

Contraindications

Those with the conditions listed below should avoid first-generation antihistamines and use second-generation antihistamines with caution:

Glaucoma.

Enlarged prostate.

High blood pressure.

Asthma/ COPD.

Hyperthyroidism.

Seizures.

Liver/ kidney disease.

Drug interactions

When antihistamines are taken with other prescription drugs, over-the-counter medications, vitamins, and supplements, it may change how they work or increase the frequency or severity of side effects. Make sure that you tell your healthcare professional about anything that you are taking to avoid any negative drug interactions, including:

Marijuana, sedatives, benzodiazepines, sleep medications, and drinking alcohol increase drowsiness when combined with antihistamines.

MAO inhibitors may increase the risk of severe side effects, including sedation, serotonin syndrome, or hypertensive crisis, when combined with antihistamines.

Antifungal medications (e.g., ketoconazole, itraconazole) and certain antibiotics (e.g., erythromycin, azithromycin) can raise the levels of some antihistamines in the blood, increasing the risk of side effects, including heart issues.

Antacids containing magnesium or aluminum: These can make fexofenadine (Allegra) less effective if taken at the same time.

Antihistamines can decrease the therapeutic efficacy of betahistine, a medication used for Meniere's disease.