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Sedating antihistamines effect is related to

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.

Several of the older drugs, called first-generation antihistamines, bind non-selectively to H receptors in the central nervous system as well as to peripheral receptors, and can produce sedation, inhibition of nausea and vomiting, and reduction of motion sickness.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.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).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.

Diphenhydramine (Benadryl) is chemically an ethanolamine, and in addition to its role in reducing allergic reactions, may be used as a nighttime sedative, for control of drug-induced Parkinsonism, and, in liquid form, for control of coughs.

Consult more detailed references for further information.

The second-generation antihistamines bind only to peripheral HThe first-generation antihistamines may be divided into several chemical classes.

The side effect profile, which also determines the uses of the drugs, will vary by chemical class.

Oral antihistamines are available as pills, chewable tablets, orally disintegrating tablets, capsules, and liquid.