Antihistamines, also called H1 blockers, are medicines that help alleviate allergy symptoms such as itchy eyes, sneezing, runny nose, and occasional itchy rashes (especially hives).
An allergy or hypersensitivity reaction occurs when the body's immune system overreacts to a substance that is normally harmless (allergen), such as mold, pollen, animal dander, or dust mites. The white blood cells of an allergic individual produce an antibody called immunoglobulin E (IgE), which attaches to the allergen. This triggers the release of histamine and other chemicals that cause allergic symptoms, such as runny nose, watery eyes, and hives. Histamine may also cause the airway passages to close making it difficult to breathe.
Antihistamines block histamine-1 receptors in the gastrointestinal tract, respiratory tract and blood vessels of an allergic individual. This prevents the release of histamine in response to an allergen. As a result, allergy symptoms are minimized.
Antihistamines are available as short-lasting or long-lasting medications. The type of medication used depends on how frequent allergy symptoms arise. Some antihistamines are available over-the-counter, while others, especially long-lasting antihistamines, are only available by prescription. The most common side effect associated with antihistamines is drowsiness. However, prescription antihistamines generally do not cause drowsiness.
While antihistamines help relieve symptoms, they do not completely control the allergic reaction because other chemicals produced by the body in response to an allergen, such as serotonin, proteases, and bradykinin generating factors, are also involved in allergic reactions.

Related Terms

Allergen, allergies, allergic reaction, allergic response, allergy, allergy symptoms, antibodies, antibody, brompheniramine, cetirizine, chemical mediators, chronic urticaria, desloratadine, diphenhydramine, drowsiness, fexofenadine, histamine-1 receptors, hives, hydroxyzine, Ig, IgE, immune, immune reaction, immune response, immune system, immunoglobulin, inflammation, inflammatory chemicals, long-acting antihistamines, motion sickness, over-the-counter antihistamines, Parkinson's disease, prescription antihistamines, sedating, short-acting antihistamines, urticaria, white blood cells.

brand names

Alavert®, Allegra®, Aller-Chlor®, AllerMax Caplets®, Aller-med®, Atarax®, Banophen®, Banophen Caplets®, Benadryl®, Benadryl Allergy®, Bromphen®, Calm X®, Chlo-Amine®, Chlorate®, Chlor-Trimeton®, Chlor-Trimeton Allergy®, Chlor-Trimeton Repetabs®, Clarinex®, Claritin®, Claritin Reditabs®, Compoz®, Contac 12 Hour Allergy®, Cophene-B®, Dexchlor®, Dimetapp Allergy Liqui-Gels®, Dinate®, Diphen Cough®, Diphenhist®, Diphenhist Captabs®, Dormarex 2®, Dramamine®, Dramanate®, Genahist®, Gen-Allerate®, Hydrate®, Hyrexin®, Hyzine-50®, Nasahist B®, Nervine Nighttime Sleep-Aid®, Nolahist®, Nytol QuickCaps®, Nytol QuickGels®, Optimine®, PediaCare Allergy Formula®, Periactin®, Phenetron®, Polaramine®, Polaramine Repetabs®, Siladryl®, Sleep-Eze D®, Sleep-Eze D Extra Strength®, Sominex®, Tavist®, Tavist-1®, Telachlor®, Teldrin®, Triptone Caplets®, Twilite Caplets®, Unisom Nighttime Sleep Aid®, Unisom SleepGels Maximum Strength®, Vistaril®, Zyrtec®.

short-acting antihistamines

Short-acting oral antihistamines, such as diphenhydramine (Benadryl®) and chlorpheniramine (Chlor-Trimeton®), are typically used to relieve mild to moderate allergy symptoms such as runny nose, sneezing, and itchy eyes. Most short-acting antihistamines are available over-the-counter. These medication typically take about 15-30 minutes to take effect, and more than one dose may be taken in a single day. Most short-acting antihistamines are taken every four to six hours, depending on symptoms. Patients should carefully read the packaging labels and follow the directions of their healthcare providers and pharmacists.
Short-acting antihistamines medications often cause drowsiness. Many short-acting antihistamines have been shown to impair cognition (slow learning) in children, even in the absence of drowsiness. Therefore, these drugs are generally not recommended in children, unless taken under the strict supervision of their healthcare providers. Loratadine (Claritin®), which is now available over-the-counter, does not cause drowsiness or affect learning in children.
Topical antihistamines like mepyramine (Antisan® cream) have been applied to the skin to relieve itching associated with various skin disorders, such as insect bites, sunburn, or poison ivy. Some antihistamines, such as Loratadine (Claritin®), hydroxyzine (Anx®, Atarax®, Atarax® Syrup or Vistaril®), cyproheptadine (Periactin®), and doxepin (Adapin® or Sinequan®), have been used to treat chronic urticaria (hives) that occurs in response to allergens.

long-acting antihistamines

Longer-acting antihistamines, including fexofenadine (Allegra®), desloratadine (Clarinex®) and cetirizine (Zyrtec®), are available by prescription to treat mild to moderate allergy symptoms. These medications are generally taken once a day to relieve long-term allergies to allergens like pet dander.
These medications cause less drowsiness than short-acting antihistamines, and they are equally effective. While these drugs usually do not interfere with learning, they should be only be used in children under the strict supervision of their healthcare providers. Common side effects may include drowsiness, dry mouth, headache, sore throat, stomach pain, vomiting, or diarrhea.

additional uses

In patients with Parkinson's disease, diphenhydramine (Benadryl®) has been used to decrease muscle stiffness and tremors, which are associated with the disease.
Also, the syrup form of diphenhydramine has been used to relieve the cough due to colds or hay fever. However, antihistamine use for common colds is controversial because studies have shown that symptoms of colds are not histamine-related.
Some antihistamines, including dimenhydrinate (Dramamine® or Driminate®), are also used to prevent motion sickness, nausea, vomiting, and dizziness.
Since antihistamines that cause drowsiness as a side effect may be used to help people fall asleep.
Hydroxyzine has been used to treat nervous and emotional conditions to help control anxiety. It has also been used to induce sleep before surgery.

side effects

Common side effects: The most common side effect of antihistamines is drowsiness. Even if this medication is taken before bed, some patients may feel drowsy or less alert when they wake up. Therefore, patients should not take this medication before driving or operating heavy machinery. Some antihistamines are more likely to cause drowsiness than others. For instance, drowsiness is less common with cetirizine (Zyrtec®) and fexofenadine (Allegra®) and rare with desloratadine (Clarinex®) and loratadine (Claritin®).
Antihistamines may also cause dryness of the mouth, nose, and/or throat. Continual dryness of the mouth may increase the chance of dental disease, including tooth decay, gum disease, and fungal infections.
Other common side effects of antihistamines include excitement, nervousness, restlessness, dizziness, confusion, painful urination, nightmares, restlessness, and irritability.
Serious side effects: Serious side effects may include seizures, hallucinations (seeing, hearing or feeling things that are not there), and mental depression. Short-acting antihistamines often cause drowsiness. Many short-acting antihistamines have been shown to impair cognition (slow learning) in children (even in the absence of drowsiness), which is why they are generally not recommended in children.
Rare side effects: Less common or rare side effects may include: fast or irregular heartbeat, fever, stomach pain, chills, clay-colored stools, dark urine, cough, diarrhea, difficulty swallowing, dizziness, fever, headache, hives, itching, prickly sensations, redness of skin, seizures, shortness of breath, skin rash, swelling, tightness in the chest, tingling sensations, unusual tiredness or weakness, wheezing, sore throat, unusual bleeding or bruising or swelling of the eyes, eyelids, face, lips, and/or tongue.
Signs of overdose: Symptoms of an overdose may include clumsiness or unsteadiness, seizures, severe drowsiness, lightheadedness, flushing or reddening of face, hallucinations, shortness of breath or trouble breathing, trouble sleeping, or severe dryness of the mouth, nose, or throat.

pregnancy and breastfeeding

General: Most, but not all, antihistamines are considered safe during pregnancy, according to the U.S. Food and Drug Administration (FDA). Commonly used over-the-counter antihistamines that are considered safe for pregnancy include chlorpheniramine (Chlor-Trimeton®), diphenhydramine (Benadryl®), and loratadine (Claritin®). In addition, celtirizine (Zyrtect®) is a commonly prescribed antihistamine that is considered safe for pregnancy. Patients who are pregnant, thinking of becoming pregnant, or breastfeeding should consult their healthcare providers before taking antihistamines.
Since small amounts of antihistamines are able to pass into the breast milk, use is not recommended in breastfeeding women. Babies are the most susceptible to side effects from the medication. Also, antihistamines tend to decrease secretions in the body and breastfeeding mothers who are taking the medications may experience a reduction in breast milk production.
Precautions: Hydroxyzine (Anx®, Atarax®, Atarax® Syrup, or Vistaril®) is not recommended for use during the first few months of pregnancy because extremely high doses have caused birth defects in animal studies. Therefore, this medication should not be taken during the first trimester of pregnancy.
Desloratadine (Clarinex®) and fexofenadine (Allegra®) have not been studied in pregnant women. However, extremely high doses of these medicines caused birth defects in animal studies. These medications may not be safe during pregnancy.
Azatadine (Optimine®), brompheniramine (Dimetapp®), cetirizine (Zyrtec®), chlorpheniramine (Aller-Chlor®), clemastine (Tavist®), cyproheptadine (Periactin®), dexchlorpheniramine (Polaramine®), dimenhydrinate (Dramamine® or Driminate®), diphenhydramine (Benadryl®), doxylamine (Nighttime Sleep Aid®), and loratadine (Clarin®) have not been studied in pregnant women. However, these drugs have not been shown to cause birth defects or other problems in animal studies. Until further research is conducted to determine the safety of these drugs, they should not be taken in patients who are pregnant or plan to become pregnant.


In general, children are more sensitive to the effects of antihistamines than adults. Serious side effects, such as seizures, hallucinations, and depression, are more likely to occur in younger patients than adults. Also, nightmares or unusual excitement, nervousness, restlessness, or irritability may be more likely to occur in children.
Short-acting antihistamines often cause drowsiness and many have been shown to impair cognition (slow learning) in children (even in the absence of drowsiness). These drugs are generally not recommended in children, unless under the strict supervision of their healthcare providers. However, loratadine (Claritin®), which is available over-the-counter, does not cause drowsiness or affect learning in children.
Due to the lack of clinical evidence and reports of side effects, many physicians do not recommend antihistamines for children.

older adults

Elderly patients are usually more sensitive to the effects of antihistamines. Confusion, difficult or painful urination, nightmares, unusual excitement, nervousness, restlessness, irritability dizziness, drowsiness, feeling faint, or dry mouth, nose, or throat may be more likely to occur in elderly patients.

possible interactions

General: Patients should tell their healthcare providers if they are taking any drugs (prescription or over-the-counter), herbs or supplements as they may interact with antihistamines.
Anticholinergics: Patients who are taking both anticholinergics (drugs used to treat stomach cramps) and antihistamines are more likely to experience side effects, such as dry mouth.
Antidepressants: Antidepressants like maprotiline (Ludiomil®) may worsen side effects associated with antihistamines, especially drowsiness and dry mouth.
Central nervous system (CNS) depressants: Central nervous system (CNS) depressants, such as diazepam (Valium®), mephobarbital (Mebaral®), and pentobarbitalsodium (Nembutal®), may worsen side effects associated with antihistamines, especially drowsiness.
Erythromycin: An antibiotic called erythromycin (like E-Mycin®) should not be taken with antihistamines because patients may experience a worsening of side effects associated with antihistamines.
Ketoconazole: Taking ketoconazole (Nizoral®), a drug used to treat fungal infections, with fexofenadine (Allegra®) may cause an increased amount of fexofenadine in the blood.
Monoamine oxidase inhibitors (MAOIs): Monoamine oxidase inhibitors (MAOI), such as isocarboxazid (Marplan®), isocarboxazid (Marplan®), phenelzine (Nardil®), procarbazine (Matulane®), selegiline (Eldepryl®) and tranylcypromine (Parnate®), should not be taken with antihistamines. MAOIs may increase the likelihood and severity of side effects associated with antihistamines.