Sharjah-based dermatologist Dr Minal Patwardhan puts the spotlight on "miracle" drugs and treatments every fortnight. Today: antihistamines.


What is an allergic reaction?

Almost every second patient we see here suffers from some sort of allergy or the other. This is actually a fall back of urbanisation and economic progress because we are constantly exposed to more and more artificial environments.

What happens in an allergic attack?

A release of histamine (endogenous amine) mediated through various allergic reactions or exposure to chemicals causes pharmacologic effects of varying intensity throughout the body.

These reactions range from mild irritation, skin allergies, eczemas, hives, swellings, sneezing, runny nose, watery eyes and itching to shock (anaphylaxis) and death.

Histamine also appears to have a neurotransmitter effect in the central nervous system (CNS).

Histamine interacts with certain receptors of the cells in the target tissue, subdivided into H1, H2 and H3.

These interactions set off a series of inflammatory reactions in the body.

What are antihistamines?

An antihistamine acts as a competitive antagonist by occupying the histamine receptor on effector cells.

However, it does not prevent the release of histamine, nor does it destroy histamine.

What this means is that once released the histamine will take some time to be controlled but further progress of the reaction is stopped. That's why it takes at least two days for allergies and itching to reduce inspite of medicines.

Histamine acts on three distinct receptors viz H1, H2 and H3. Bronchial (lungs and windpipe) and intestinal smooth muscle contraction and increased capillary oozing are mediated by H1 receptors and antagonised by H1 receptor blocking agents

The action on the gastric (stomach) secretion and the heart contraction is mediated via H2 receptors.

What are the effects other than anti-allergy?

The first generation H1 antihistamines can stimulate and depress the CNS... decreasing alertness, slow reaction times, blurred vision or double vision, fatigue and sleep are common side effects.

Therefore some antihistamines are sold as sleep-aids.

Stimulation of the CNS includes restlessness, nervousness, fits, euphoria and sleeplessness which may be seen in patients taking high doses.

The more recently introduced second generation H1 antihistamines do not cross the blood brain barrier and therefore are free of CNS side effects.

These include terfenadine, astemizole, loratadine, cetirizine, fexofenadine, desloratadine among others.

Azelastine (Allergodil) and levocabastine (Livostin) are available as nasal sprays.

Some H1 antihistamines can suppress motion sickness but are less effective against an episode already present.

Dry eyes, dry mouth are also common side effects.

The new second generation antihistamines have less drying activity.

The duration of action of most of the first generation ones is around four hours. In the elderly metabolism may be slower so care has to be taken during administration.

Side effects like excitation, convulsions, arrhythmia with possible sudden death may be observed in patients with liver disease or as an interaction with other drugs.

Because of this, two drugs (terfenadine and astemizole) have been removed from the market.

The common H2 antihistamines are cimetidine , ranitidine , famotidine and nizatidine.

Adverse effects are seen mainly with cimetidine and include dizziness, confusion, skin rashes, muscle pains and impotence.

• Never use medication when you are driving or working because unless tried and tested, however 'non-sedating' it is, the medication can still cause drowsiness and disorientation.

— With inputs from Dr. Mustafa Mahesri, Al Deyafa Medical Centre, Sharjah