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The Side Effects of Antihistamine drugs

How antihistamines act

How antihistamines act

Antihistamines are type of medications used for allergic reactions, rhinitis, and common cold. Most common feature of all antihistamines is blockade of histamine type 1 receptors. They are an ingredient in most over the counter cough syrups. However, they can cause some side effects.

Antihistamines can be divided into first generation and second generation. First generation anti histamines crosses the blood brain barrier and they can cause more side effects. Second generation or newer antihistamines does not crosses the blood brain barrier and they cause less side effects.

Commonly available first generation antihistamines are Chlorpheniramine (Piriton, Histop), Promethazine (Phenergan), and Dephenhydramine (Benadryl). Most of the old antihistamines are available in over the counter medications.

Common second generation antihistamines are Cetirizine, Loratidine, Fexofenadine (Telfast) and Levocetirizine.

Antihistamines are available as wide tablets, injections, skin creams, eye drops and nasal drops.

What are the side effects of antihistamines?

Most common side effect is drowsiness and sedation. They are much more common in older first generation anti histamines. However, cetirizine (second generation) is more sedating than other newer antihistamines. Other second generation antihistamines are less sedating than cetirizine and older antihistamines. Due to this side effect, it is not prudent to drive motor vehicles or operate machinery with these drugs until the patient knows how it affects him.

Older antihistamines can cause headache, psychological impairment, urinary retention, blurred vision and dry mouth. In addition they can cause constipation.

Other rare side effects of antihistamines are hypotension, palpitation, cardiac rhythm problems, dizziness, confusion, depression, sleep problems, tremors, fits, allergic reactions (difficulty in breathing, edema of mouth and tongue, and anaphylaxis, rashes, and photosensitive rashes), liver problems, and glaucoma.

Use of antihistamines is pregnancy and breast feeding

Most of the antihistamine manufactures advice not to use these drugs in pregnancy and breast feeding. However, except Loratidine there is no documented evidence of fetal harm. In addition, most of the antihistamines are secreted in breast milk.

Liver disease

Antihistamines, especially older ones should not be used in severe liver failure. It is because they tend to cause drowsiness.

If you have following conditions then do not use older antihistamines, even though they are freely available.

1. prostatic hypertrophy

2. urinary retention

3. glaucoma

4. intestinal obstruction

5. epilepsy

6. porphyria


Antihistamines are common medications used for allergies, common cold and cough. Older antihistamines cause more side effects. However, ironically most of the older antihistamines are available over the counter. Therefore you need some knowledge before taking a cough syrup.


British National Formulary

Warning and disclaimer

This article is for the general information only. You should consult your doctor if you are to take prescription drugs.

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Wijesiri Muththettowage on July 19, 2019:

Very helpful

Thanks God bless

Dr Manura Nanayakkara (author) from Sri Lanka on June 30, 2011:

Thanks for the comments

Ruby Jean Richert from Southern Illinois on June 21, 2011:

Very helpful. Thank you.