Antihistamines: Uses and Effects | Drugs | Pharmacology

In this article we will discuss about:- 1. Pharmacokinetics of Antihistamines 2. Pharmacological Actions of Antihistamines 3. Therapeutic Uses 4. Adverse Effects.

Currently more than 24 antihistamine drugs are available. All antihistamines are of potential value in the treatment of allergies and choice depends on their duration of action, incidence of sedation and anticholinergic actions.

They are generally divided on the basis of their sedating action into sedating and non-sedating antihistamines:

Highly Sedating – Alimemazine (trimepyrazine), diphen­hydramine (benadryl) and promethazine (phenergan).

Moderately Sedating – Chlorpheniramine, cyclizine, cryptoheptadine (periactin), mequitazine and pheniramine (Avil).

Non-Sedating – Terfenadine, cetrizine, fexofenadine, loratidine, and cinnarizine.

Pharmacokinetics of Antihistamines:

All antihistamines are rapidly and completely absorbed from GIT. Antihistamines differ in their duration of action; many sedating antihistamines are relatively short acting, while most of the non-sedating ones are long acting. Antihistamines are metabolized by hydroxylation.

Pharmacological Actions of Antihistamines:

Antihistamines block the actions of histamine on bronchial and intestinal smooth muscles and small blood vessels. They block the sensory impulses responsible for itching. Sedation is a common effect. The non-sedating antihistamines cause less sedation and psychomotor impairment, because they penetrate the blood-brain barrier only to a slight extent. Antihistamines possess anticholinergic properties and have local anesthetics and antipruritic effects.

Therapeutic Uses of Antihistamines:

i. Allergic disorders:

All antihistamines are effective in the treatment of urticaria, serum sickness, pruritis, insect bites and stings, seasonal allergic rhinitis (hay fever) and drug fever. Chlorpheniramine intravenously is a useful adjunct to adrenaline in the emergency treatment of anaphylaxis and angioedema. Topical preparations such as eye drops containing antazoline (antistin) is used for allergic conjunctivitis. Antihistamines are of no value in bronchial asthma.

ii. Vestibular disorders:

Antihistamines are slightly less effective, but are better tolerated in vertigo, tinnitus, nausea and vomiting in Meniere’s disease and motion sickness. Sedating antihistamines are used.

Adverse Effects of Antihistamines:

Side effects common with sedating antihistamines are drowsiness, headache, psychomotor and anticholinergic effects such as urinary retention, dry mouth, blurred vision and GIT disorders. Terfenadine, rarely, can cause dangerous cardiac arrhythmias.