What are antihistamine drugs?

Antihistamine drugs are medications that block histamine, a chemical your immune system releases during an allergic reaction that causes sneezing, itching, hives, and a runny nose. They come in two main generations: first-generation drugs like diphenhydramine, and second-generation drugs like loratadine, cetirizine, and fexofenadine. Antihistamines relieve allergy symptoms but do not treat the underlying cause. Some first-generation antihistamines are also used for motion sickness or as a short-term sleep aid.

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.

Diphenhydramine is the generic name for Benadryl, a first-generation antihistamine used to relieve sneezing, itching, hives, and a runny nose from allergies. It crosses into the brain, which is why it causes significant drowsiness and is sometimes used as a short-term sleep aid. It works quickly but typically lasts only four to six hours, requiring more frequent dosing than newer antihistamines.

Cetirizine is a second-generation antihistamine sold under the brand name Zyrtec. It blocks histamine with much less drowsiness than diphenhydramine and typically lasts about 24 hours with once-daily dosing. Cetirizine and its more potent relative levocetirizine can still cause mild drowsiness in a minority of users, more so than loratadine or fexofenadine. It's commonly used for ongoing seasonal or year-round allergy symptoms.

It depends on the generation. First-generation antihistamines like diphenhydramine cross into the brain and commonly cause significant drowsiness. Second-generation antihistamines such as loratadine and fexofenadine cause little to no drowsiness for most people, though cetirizine and levocetirizine cause mild sleepiness in some users. If daytime drowsiness is a concern, a non-sedating option like fexofenadine is usually preferred.

Common second-generation antihistamines include loratadine (Claritin), cetirizine (Zyrtec), fexofenadine (Allegra), and levocetirizine (Xyzal) — all taken once daily with little drowsiness. The main first-generation example is diphenhydramine (Benadryl), which works fast but causes significant sedation. Chlorpheniramine is another older first-generation option found in some combination cold products. A pharmacist can help match the right drug to your symptoms.

The most common side effect, especially with first-generation drugs like diphenhydramine, is drowsiness; dry mouth, blurred vision, and constipation can also occur. Second-generation antihistamines cause far fewer side effects, though cetirizine and levocetirizine can still cause mild drowsiness in some people. Talk to a pharmacist or clinician if side effects are bothersome or persistent.