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The Use of Antihistamines

Last week we discussed the abundance of ragweed and its associated effects. This week we will discuss the use of antihistamines, of the first generation. Many of us “seasoned” pharmacists remember when these drugs were moved from behind the counter to OTC status. Although safe and effective, pharmacist expertise is necessary in quite a few patient groups.

A brief HISTORY of antihistamines:

  • 1942 Bernard Halpbern introduced the first antihistamine: N-diethylaminoethyl-N-benzylalanine- Institute Pasteur in Paris
  • 1945 diphenhydramine became available, followed by chlorpheniramine, bromphineramine and promethazine later in the 1940’s. Chlorpheniramine and Bromphiramine became over the counter September 9, 1976.
  • Diphenhydramine first as an antitussive in Aug-1981 (Benylin); as a sleep aid (50mg) in 1982 (Sominex-2), and as an antihistamine (25mg) Jan-1985 (Benadryl)

MECHANISM: H1-antagonists competitively inhibit the action of histamine on tissues containing H1-receptors. Some also block histamine release, but only in excessive doses. The H1-antagonists do not block antibody production or antigen-antibody interactions.

USES: Symptomatic treatment (sneezing, rhinorrhea, and itching of eyes, nose, and throat) of allergic rhinitis, chronic idiopathic urticarial, as well as motion sickness & nausea/vomiting.

THREE GENERATIONS OF OTC ANTIHISTAMINES:

  • First generation are the older, and more sedating They also cause numerous anti-cholinergic side effects. They cross the blood brain barrier.
    • Examples: Chlorpheniramine (Chlor-Trimeton), Clemastine (Tavist), Diphenhydramine (Benadryl), Brompheneramine (Dimetane)
    • Anticholinergic side effects: blurred vision, dry mouth, urinary retention, constipation. (“can’t see, can’t spit, can’t pee, can’t sh*t”)
  • The second generation (non-sedating) do NOT cross the blood brain barrier, causing minimal sedation, and NO anticholinergic side effects.
    • Examples: Loratadine (Claritin) and Cetirizine (Zyrtec)
  • Third generation are the “active enantiomers” of the second generation antihistamines
    • Fexofenadine (Allegra)

Have a great day on the bench!

Pete Kreckel –Thompson Pharmacy

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