22.1 One of the Workhorses of Allergy Medications

Antihistamines are the major workhorse medications in the treatment of allergic disorders. This chapter will review oral H1 and H2 antihistamines, as well as intranasal and ocular antihistamines.

22.2 What Is This Class of Medication?

Histamine is one of the primary mediators of the allergic cascade and responsible for symptoms of sneezing, itching, nasal congestion, and rhinorrhea, among others. Histamine release is what generates the wheal and flare reaction seen in skin testing. Antihistamines work by blocking the binding of histamine to receptors. H1 antihistamines block binding to H1 receptors, and H2 antihistamines block binding to H2 receptors.

H1 antihistamines are further divided into first-generation and later-generation antihistamines. The first-generation H1 blockers cross the blood-brain barrier to a greater degree than later-generation H1 blockers. This leads to more frequent cognitive side effects, as well as some anticholinergic side effects, such as dry mouth. They also act as inhibitors of some liver enzymes, such as CYP2D6, which may cause them to effect the metabolism of other drugs. For this reason, first-generation antihistamines are not recommended for the treatment of allergic rhinitis.

H2 antihistamines have a larger impact on gastric acid secretion than on allergic symptoms and their usefulness in treating allergic rhinitis has not been definitively proven. However, they may impact skin testing and occasionally, some practitioners will use H1 and H2 antihistamines in combination to treat allergic rhinitis.

22.3 What Symptoms Are Good for Treating?

Oral H1 antihistamines (all generations): Sneezing, itching, rhinorrhea, and hives; can be used for ocular itching, redness, and tearing, but intranasal antihistamines have a better effect on these symptoms.

Oral H2 antihistamines: Clinical benefit in allergic rhinitis is unclear.

Intranasal antihistamines: Nasal congestion, sneezing, nasal itching, rhinorrhea, eye symptoms (itching, redness, conjunctival edema, tearing). These sprays have a rapid onset of action (as quickly as 15 minutes).

Ocular antihistamines: Eye itching, redness, and conjunctival edema.