What are antihistamine drugs?
Short answer
Medication question
Antihistamine drugs block histamine, the chemical behind allergic reactions. H1 antihistamines treat allergies and include sedating first-generation types like diphenhydramine (Benadryl) and non-drowsy second-generation types like loratadine, cetirizine, fexofenadine, and levocetirizine. H2 antihistamines like famotidine reduce stomach acid instead.
This is a general research summary, not medical advice. A pharmacist can confirm it for your situation.
Key facts
H1 antihistamines block histamine at H1 receptors to relieve allergy symptoms.
Non-drowsy second-generation antihistamines include loratadine, cetirizine, fexofenadine, and levocetirizine.
Antihistamines are a common medication for managing allergies.
Here's the nuance
A one-liner rarely tells the whole story. This is the honest detail behind the short answer above.
Antihistamine drugs are medicines that block histamine, a chemical the body releases during allergic reactions. Histamine triggers sneezing, a runny nose, itching, watery eyes, and hives, so blocking its receptors reduces those symptoms. The antihistamines people use for allergies act at H1 receptors and are known as H1 antihistamines.
H1 antihistamines come in two generations. First-generation (sedating) drugs cross into the brain and cause drowsiness; examples include diphenhydramine (Benadryl), chlorpheniramine, hydroxyzine, and doxylamine. Second-generation (non-drowsy) drugs cause much less drowsiness and usually last about 24 hours; examples include loratadine (Claritin), cetirizine (Zyrtec), fexofenadine (Allegra), and levocetirizine (Xyzal). Second-generation options are generally preferred for everyday allergy control, while first-generation drugs are useful for short-term or nighttime relief and for uses like motion sickness and sleep.
There is also a separate group called H2 antihistamines, such as famotidine (Pepcid). These block histamine at H2 receptors in the stomach to reduce acid and treat heartburn, so they are technically antihistamines but are not used for hay fever or allergy symptoms.
Antihistamines relieve symptoms but do not cure allergies, and they do not treat anaphylaxis, a severe reaction that requires epinephrine and emergency care. For people whose allergies are not controlled by medication, allergen immunotherapy is the only treatment that reduces sensitivity at its source rather than just masking symptoms. Read labels carefully, since many combination cold and allergy products contain an antihistamine, and ask a pharmacist which option fits your needs.
The key points
The handful of things that decide the answer — skim these if you only read one section.
They block histamine
Antihistamine drugs stop histamine from binding to its receptors, easing allergic symptoms.
First-generation (sedating)
Examples include diphenhydramine (Benadryl), chlorpheniramine, hydroxyzine, and doxylamine; they cause drowsiness.
Second-generation (non-drowsy)
Examples include loratadine (Claritin), cetirizine (Zyrtec), fexofenadine (Allegra), and levocetirizine (Xyzal).
H2 antihistamines are separate
Famotidine (Pepcid) blocks H2 receptors to reduce stomach acid, not to treat allergies.
They relieve, not cure
Antihistamines ease symptoms but do not cure allergies or treat anaphylaxis, which needs epinephrine.
Please read before acting on this
General information — not medical advice
This page is a general research summary about the medication in question. It does not account for your specific health, other medicines you take, or your dose, and it can go out of date. It is not medical advice, a prescription, or a substitute for professional judgment.
Ask your pharmacist or prescriber before starting, stopping, changing, or continuing any medication — they can check it against your situation in minutes.
❓Frequently Asked Questions
First-generation antihistamines like diphenhydramine cross into the brain and commonly cause drowsiness, and they are short-acting. Second-generation antihistamines like loratadine, cetirizine, and fexofenadine cause much less drowsiness and usually last about 24 hours. For daytime, ongoing allergies, second-generation drugs are generally preferred. A pharmacist can help you choose.
Yes, technically. Famotidine (Pepcid) is an H2 antihistamine that blocks histamine at H2 receptors in the stomach to reduce acid and treat heartburn. It is a different class from the H1 antihistamines used for allergies and does not relieve sneezing or a runny nose. Choose the type that matches your symptoms.
No. Antihistamines relieve symptoms by blocking histamine, but they do not cure the underlying allergy or change how sensitive you are. For people whose allergies are not controlled with medication, allergen immunotherapy is the only treatment that reduces sensitivity at the source. Ask a clinician whether immunotherapy is appropriate for you.
The commonly used non-drowsy antihistamines are second-generation drugs: loratadine (Claritin), fexofenadine (Allegra), cetirizine (Zyrtec), and levocetirizine (Xyzal). Cetirizine and levocetirizine can cause mild drowsiness in some people. They are typically taken once a day. Read the label and ask a pharmacist if you are unsure which is best.
Reviewed by
Curex Editorial TeamReviewed for accuracy
Last reviewed August 2026
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This content is for general informational purposes only. It is not medical advice, a prescription, or a substitute for professional judgment, and it does not account for your health, other medications, or dose. Always ask your pharmacist or prescriber before starting, stopping, or changing any medication.