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- What are oral antihistamines?
- What are examples of oral antihistamines available in the US?
- What are the side effects of oral antihistamines?
- What drugs interact with oral antihistamines?
- What formulations of oral antihistamines are available?
- What about taking oral antihistamines during pregnancy or while breastfeeding?
What are oral antihistamines?
The term, antihistamine generally is used for medications used to treat various symptoms believed to be caused by histamine, for example, as part of the common cold, including:
(A second type of antihistamine is used primarily for suppressing acid production in the stomach and treating acid-related diseases such as ulcers of the stomach. These antihistamines will not be discussed further.)
Antihistamines also may be used to treat motion sickness, insomnia (difficulty sleeping), and anxiety. Antihistamines work by blocking the effects of a chemical called histamine that is responsible for many allergic symptoms.
Patients who experience significant allergic symptoms regularly may take daily antihistamines to keep their symptoms under control. Antihistamines also can be used on an as needed basis for those who experience occasional symptoms or symptoms triggered by exposure to certain irritants such as animal hair, plants, medications, and food products. Some antihistamines also may be used occasionally to help with sleep.
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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