Antihistamines are a class of drugs that block the physiological action of the chemical histamine in the body to neutralize histamine-induced allergic reactions that can include fever, itching, sneezing, a runny nose, and watery eyes. They, therefore, form the main component in the treatment of allergies.


Antihistamines form the mainstay of all anti-allergy treatments playing an important role in getting rid of your allergy symptoms and giving you relief.

They are thus used in the treatment of:

  • Cold
  • Flu
  • Sneezing
  • Seasonal allergic rhinitis (hay fever)
  • Allergic sinusitis
  • Cough due to allergic bronchitis
  • Bronchial asthma
  • Watery and itchy eyes
  • Itching in the nose and the throat
  • Allergic skin conditions such as itching (pruritis), skin rash, hives (urticaria), eczema, and psoriasis
  • Insect bites and bee stings
  • Stomach upset and pain due to food allergy
  • Histamines cause drowsiness and, therefore, help to induce sleep in treating insomnia
  • As a supplement in the treatment of motion sickness
  • In the treatment of anaphylactic shock

Antihistamines are available as tablets or pills, capsules, liquids, nasal sprays, eye drops, and injectables. You can take them twice or thrice a day depending on the severity of the hypersensitivity attack.

How antihistamines act

To understand how these drugs work, you should first know why you get allergy. As a result of the allergen and the IgE antibody reaction in the body, the mast cells release histamine, which acts on the receptors in the body.

Mast cells are a type of white blood cell that are present in connective tissues throughout the body, especially under the skin, near blood vessels and lymph vessels, in nerves, and in the lungs and intestines.

Histamine is the main chemical that is responsible for the allergy reaction and the ensuing symptoms.

And as the name suggests, antihistamines oppose or block the action of the histamines at the receptor sites (named H1 to H4) and stop the allergen and antibody reaction, leading to the regression of the symptoms.

The receptors on which histamine acts are classified as H1, H2, H3, and H4. Each has its specific place in the body. They are present in the brain, the nervous system, the skin, the stomach, the intestines, and so forth.

The action of histamine upon the particular receptors depends on where the histamine is released in the body during the reaction.  For example, a food allergy will release the histamine in the stomach and the intestines giving rise to symptoms of food allergy.

By their blocking action on histamine at the receptor sites, antihistamines are able to neutralize its action and clear the allergy symptoms.

How quickly do antihistamines work?

Oral antihistamines start to act about 30 minutes after being ingested. The peak of the action occurs after about 1 to 2 hours after they are taken. Antihistamine nasal sprays work faster, but their action is limited to the nose.

Intramuscular injections work faster and will take effect quickly, in about 1 to 2 hours, and clinical signs should improve around that time. With IV administration, the onset of action is usually within 1 minute, with a peak action at 2 to 6 minutes.

Side effects

Antihistamines have one notable side effect: They cause drowsiness and therefore, you should use them with caution, especially when driving, riding, or when operating heavy machinery.

This side effect of drowsiness is due to the fact that antihistamines are able to cross the blood-brain barrier and bind with the H1 histamine receptors in the brain cells.

Other mild side effects include dryness of the mouth and the eyes, and headache. Occasionally, you may experience a faster heart rate and males may develop urinary retention.

Some brands claim to be non-sedating and this claim, while true in some people, may still cause drowsiness in others. Be careful, therefore, not to drive or ride a vehicle after taking this drug. Make sure they do not make you drowsy before doing so.

OTC antihistamine examples

Some of the antihistamine medicines are available over the counter (OTC) while some need a prescription. The list is long but to name a few with their brand names:

Examples of over-the-counter antihistamines include:

  • Brompheniramine (Dimetane)
  • Diphenhydramine HCl (Benadryl)
  • Chlorpheniramine maleate (Chlor-Trimeton)
  • Fexofenadine (Allegra)
  • Cetirizine (Zyrtec)

Examples of prescription antihistamines include

  • Cyproheptadine
  • Azelastine nasal spray (Astelin)
  • Azelastine eye drops (Optivar)
  • Carbinoxamine (Palgic)

Classification of antihistamines

Antihistamines are classified into two groups:

The first generation of antihistamines

The first-generation antihistamines are those, which include the older ones, which cause drowsiness or sedation. They do this because they are able to cross the blood-brain barrier and act on the histamine H1 receptors in the brain to block the action of histamine. They are, therefore, called H1 receptor antagonists or H1 blockers.

Examples of first-generation antihistamines:

  • Promethazine
  • Alimemazine
  • Dexchlorpheniramine
  • Brompheniramine
  • Buclizine
  • Carbinoxamine
  • Doxylamine

First-generation antihistamines can cause antimuscarinic side effects by which they relax the smooth muscles and cause decreased secretion of saliva, sweat, and digestive juices.

Side effects due to antimuscarinic properties include:

  • dry mouth due to decreased secretion of the saliva
  • blurred vision due to dilation of the pupil
  • constipation due to relaxation of the smooth muscles of the bowels
  • retention of urine due to relaxation of the smooth muscles of the urinary bladder

Not all first-generation antihistamine drugs exhibit antimuscarinic or anticholinergic properties. For example, Oxatomide is a first-generation sedating antihistamine without antimuscarinic effects.

The second-generation antihistamines

The second-generation antihistamines were introduced to remove the side effect of drowsiness found with the older first-generation drugs.

They keep the patient alert because they do not cross the blood-brain barrier to an extent as much as the older antihistamines did. To a great extent, this was successful in most people, but some people did experience some drowsiness.

The second generation type of antihistamines last in the blood for a longer time and if taken in higher doses they can induce drowsiness.

Terfenadine and astemizole were the first second-generation antihistamines to be introduced, but they produced side effects of ventricular arrhythmias. They were withdrawn from production.

Currently, mequitazine, cetirizine, loratadine, mizolastine, fexofenadine are marketed and they do not produce sedation because they do not cross the blood-brain barrier. Their action is and remains peripheral.

Of those mentioned above, mequitazine exhibits antimuscarinic properties and you can, therefore, experience side effects of these properties mentioned above.

Contraindications for antihistamines

People with certain conditions may not be advised these drugs. These conditions include:

  • a type of metabolic disorder (acute porphyria)
  • during pregnancy
  • during lactation
  • prostate enlargement (benign prostatic hyperplasia)
  • acute glaucoma, a condition where the pressure in the eye is raised
  • when you’re trying to conceive

Drug interactions

Practice caution and close monitoring if you are taking antihistamines with antidepressants and/or anti-fungal drugs.  Seek your doctor’s opinion.

Antihistamines and alcohol

Since first-generation antihistamines cause drowsiness, visual disturbances, and heart palpitations, consuming alcohol when you are under the effect of this drug, can increase the risk and depth of these side effects.

Second-generation antihistamines do not cause drowsiness and consuming alcohol while on these drugs may not increase sedation, but there are other side effects like heart problems due to drug interaction, that contraindicate taking alcohol while on these drugs.

However, some experts feel that if you take the second-hand generation antihistamines within the prescribed dose, there is no harm if you take alcohol.

The bottom line is that you should restrict the amount of alcohol to the prescribed level of not more than two drinks (60 ml equals one drink), drunk well diluted and over time. This will keep the level of alcohol in the blood to well below the dangerous levels to cause any dangerous drug interaction.

Do antihistamines raise blood pressure?

Patients with high blood pressure can safely use antihistamines to relieve the symptoms of allergy because they do not cause any effect on the blood pressure.

If you have nasal congestion, local nasal corticosteroids will help clear the congestion.

However, decongestants, which are also used to relieve allergy discomfort are contraindicated in patients with high blood pressure, rhythm problems (palpitations), or those who have severe blockages in the blood vessels of their heart. They can increase blood pressure and can accelerate the heart rate.

Therefore, though you can use Allegra tablet for your allergy, you should not use Allegra-D, which is an antihistamine combined with a decongestant.

Antihistamines and fertility

Histamines play an important role in releasing the ovum (egg) from the ovary and implanting it in the womb.

Taking antihistamines when trying to get pregnant can block the action of the histamines and interfere with this process and prevent you from conceiving.

Whether you are taking Allegra for allergy or Benadryl for a cough and cold, avoid using these drugs when you are planning to get pregnant.

At such times, go in for natural remedies to solve your allergy problems.

Antihistamines, such as chlorpheniramine maleate, are also contraindicated in pregnancy because they can cause premature birth and can lead to blindness in the baby due to retinopathy.