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Clinician’s Corner: Antihistamines

Last week we discussed the abundance of ragweed and its associated effects right here in Central Pennsylvania.  This week we will discuss the use of antihistamines, of the first generation.

MECHANISM: ANTI-histamines block the action of histamine on tissues containing histamine receptors.  Some also block histamine release, but only in excessive doses.

USES: Antihistamines treat symptoms (sneezing, rhinorrhea, and itching of eyes, nose, and throat) of allergic rhinitis, chronic skin itching , as well as motion sickness & nausea/vomiting.  Always best to start antihistamine therapy at least two hours before exposure.


First generation are the older, and more sedating antihistamines. They also cause numerous anti-cholinergic side effects. They cross the blood brain barrier.

  • Examples: Chlorpheniramine (Chlor-Trimeton), Clemastine (Tavist), Diphenhydramine (Benadryl), Brompheniramine (Dimetane)
  • First generation antihistamines are commonly used for allergy, hives, sleep induction, cough, dizziness, motion sickness, itching, adjunct for nausea and vomiting and runny nose. A lot of the benefits of these first-generation antihistamines is tied to their anti-cholinergic side effects.
  • Anticholinergic side effects: blurred vision, dry mouth, urinary retention, constipation.


  • AVOID in ELDERLY:. Due to their highly anticholinergic activity and reduced clearance with advanced age. Tolerance develops when used as hypnotic; may also see increased risk of confusion, dementia, cognitive impairment, dry mouth, and constipation. May also increase fall risk
  • Avoid in men with BPH (prostate hypertrophy)
  • Avoid in children under age six

The second generation (non-sedating) do NOT easily pass into the brain, 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)

We will explore the second and third generation antihistamines next week.

Until we get two “killing frosts” in October we will be doing battle with ragweed.  Your Thompson Pharmacist can help you select an antihistamine that is best for you.  Picking an antihistamine is easy.. picking the RIGHT antihistamine requires a consult from your Thompson Pharmacist.  Go Ahead and ASK… at Thompson Pharmacy it’s all for YOU!!!