Antihistamines: List, Types, & Side Effects

What Is Histamine?

Histamine is synthesized in the body from an amino acid known as histidine.It is mainly distributed in the following tissues:

  • Basophils
  • Mast cells
  • Neurons of the brain
  • Enterochromaffin like cell in the stomach.

Allergy or inflammation causes the release of histamine from mast cells.

There are Four Types Of Histamine Receptors In In The Body i.e H1, H2,H3,H4.

H1 Receptor Are distributed in the body in the ;

  • Smooth Muscle
  • Brain
  • Endothelial cells of blood vessels.

H2 Receptors are distributed in;

  • Parietal cells of stomach
  • Mast Cells
  • Brain
  • Heart Muscle

H3 Receptors are presynaptic and they are distributed in

  • Brain
  • Myenteric Plexus Of GIT
  • Other Neurons

H4 Receptors are mainly distributed on leukocytes,both in the blood and bone marrow.Stimulation of H4 Causes the chemotatic effects on the eosinphils and mast cells and these cells are attracted to the site of inflammation and allergic reations.


  • Antihistamines are Important medicines often used In the treatment of allergies, such as Allergic Rhinitis(hay fever).Antihistamine are usually used topically on the skin ,in the eye and in the nose.
  • Oral antihistamine are also very important in the prevention of urticaria,and in the treatment of pruritus ,urticarial rashes insect bites and stings.
  • Antihitamines like cyclizine, promethazine teoclate and cinnarizine are also sometimes used to prevent motion sickness and antihistamines are also used for short-term treatment for sleeping difficulties (insomnia).
  • Antihistamine May Be Sedating and Non-Sedating.
  • All old antihistamines are somewhat more sedating as compare to newer agents.chlorphenamine and Cyclizine are less sedating as compare to promethazine and alimemazine.
  • Non Sedating Antihistamine Include Cetrizine,Levocetrizine.Loratdine,Desloratidine,Fexofenadine,Acrivastine and Bilastine ,Rupatidine and Mizolasttine Cause Less Sedation.
  • Most antihistamines can be bought from pharmacies and shops, but some are only available on prescription.

Antihistamines (H1 Antagonists)

They Block H1 Receptors Of Histamine.


Antihistamines are broadly divided into two groups:

1st generation antihistamines And 2nd generation antihistamines

First Generation Drugs

  • Ethanolamines
  • Carbinoxamine maleate
  • Clemastine fumarate
  • Dimenhydrinate
  • Diphenhydramine HCl
  • Ethylaminediamine
  • Tripelennamine HCl
  • Pyrilamine Maleate
  • Antazoline
  • Piperazine Derivatives
  • Hydroxyzine HCl
  • Cyclizine HCl
  • Meclizine HCl
  • Alkylamines
  • Brompheniramine maleate
  • Chlorpheniramine maleate
  • Pheniramine
  • Phenothiazine
  • Promethazine HCl
  • Miscellaneous
  • Cyproheptadine
  • Phenindamine

Second Generation Drugs

  • Fexofenadine
  • Terfenadine
  • Loratadine
  • Cetirizine
  • Astemizole
  • Acrivastine
  • Levocabastine


There is no evidence of teratogenicity Except for Hydroxyzine Where Toxicity has been reported in animal studies with high doses.But Still, Most Manufacturers advise for avoiding in pregnancy.

Side Effects :

Some of the main side effects of antihistamines include:
• Drowsiness
• Dizziness
• Dry mouth
• Nausea and vomiting
• Trouble peeing or not being able to pee
• Blurred vision
• Confusion
• Restlessness or moodiness (in some children).

Bottom Line

Antihistamines are a class of medications used to treat allergic reactions by blocking the effects of histamine, a chemical produced by the immune system in response to an allergen. They are effective in reducing symptoms such as itching, sneezing, and runny nose, but may also cause side effects such as drowsiness, dry mouth, and blurred vision. It is important to follow dosage instructions carefully and consult with a healthcare provider before taking antihistamines, especially if you have any medical conditions or are taking other medications.

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