Many different brands and forms of oral antihistamines are available over the counter (OTC). Oral antihistamines are available as pills, chewable tablets, orally disintegrating tablets, capsules, and liquid. Some antihistamines are found in combination pills that contain other medications. For example, antihistamines are commonly combined with decongestants (for example, Claritin-D, Zyrtec-D, Allegra-D), a class of medicine that is used to dry up the nasal passages and relieve head congestion.
Antihistamines are divided into two categories, first generation or older agents and second generation or newer agents. First generation antihistamines have many drawbacks including side effects of drowsiness and significant anticholinergic side effects that, for example, can cause difficulty urinating or constipation, and are therefore not used very often. The newer antihistamines (second generation) are less likely to cause these side effects.
Antihistamines also differ in how long they work. Long-acting antihistamines provide symptom relief for up to 8-12 hours, while shorter acting agents last for up to 4 hours but begin working faster.
What are examples of oral antihistamines available in the US?
First generation antihistamines include
- chlorpheniramine (Chlor-Trimeton)
- diphenhydramine (Benadryl)
- doxylamine (found in many OTC sleep aids including Unisom)
- carbinoxamine (Karbinal ER)
Second generation antihistamines include
- fexofenadine (Allegra)
- loratadine (Claritin)
- loratadine ODT (Alavert, Tavist ND)
- desloratadine (Clarinex)
- certirizine (Zyrtec)
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