The antihistamines are competitively antagonize actions of histamine at the H1 receptors.

Clinical Classification:

  • Potent and Sedative – Diphenhydramine, promethazine.
  • Potent but Less Sedative – Crpheniraminehlo, chlorcyclizine.
  • Less Potent and Less Sedative – Pheniramine, mepyramine.
  • Non Sedative – Cetrizine, terfenadine.

Chemical Classification:

  • Ethanolamine – Diphenhydramine, Bromodiphenhydramine.
  • Ethylenediamines – Mepyramine maleate, Pyrilamine maleate.
  • Alkylamines – Chlorpheniramine maleate, Bromopheniramine.
  • Piperazines – Cyclizine Hcl, Chlorcyclizine Hcl
  • Phenothiazines – Promethazine, Trimeprazine.
  • Aniline Derivatives – Antazoline.
  • Piperidines – Terfenadine.
  • Hydroxyzine – Cetrizine.
  • Miscellaneous – Clemizole, Pyrobutamine.


Pharmacological Action of H1 Receptor Antagonists:

  • Antihistaminic Action: The antihistamines block histamine effects at a variety of sites. Thus they antagonized on the smooth muscle of the gut, the uterus and blood vessels and inhibit histamine augmented salivary secretion. Release of adrenaline to asrenal medulla in response to histamine is abolished. Constriction of larger blood vessels by histamine is also antagonized.
  • Anti allergic action: Many manifestations of immediate hypersensitivity are suppressed. Urticaria, itching and angioedema are well controlled. Anaphylactic fall in BP is only partially prevented.
  • Sedative & Hypnosis: CNS depression is a common side effect with the majority of anti histaminic drugs induces varying degrees of sedation, drowsiness and sleep. Sedation is sometimes beneficial particularly in the treatment of allergic reactions.
  • CNS stimulations: Stimulation is less commonly encountered with antihistaminic than depression conventional doses of a few drugs such as phenindamine may occasionally produce restlessness, tremors and insomnia and activate latent epilepsy.
  • Antiemetic and Anti Motion Sickness Effects: Motion sickness attributed commonly to a considerable extent by diphenhydramine, promethanine and piperanine anti histaminics. Vomiting also controlled by anti histaminics like promethanine.
  • Antiparkinsonian Effect: Central antimuscarinic actions of some antihistaminics are helpful in treating Parkinsonism.
  • Autonomic Nervous System: Majority of the antihistaminics exhibit an atropine like activity. Dryness of mouth is a common side effect. Phenindamine exert an adrenergic blocking effect.
  • Cardiovascular System: Most antihistaminics cause a fall in B.P on iv injection. However this is not eleiden on or administration.
  • Local Anaesthetics: Some antihistaminics exhibit local anaesthetic activity.


Mechanism of Action:

The antihistamines effectively block the action of exogenously administered histamine in the animal and man. H1 receptor antagonists are minimally effective against the symptoms of anaphylaxis. Generally urticaria and pruritis are antagonized by antihistamines, but bronchoconstriction and hypotension are not controlled.


The antihistaminics compounds are well absorbed in oral or parenteral administration. The compounds are mainly metabolized in the liver by hydroxylation and may stimulate the hepatic microsomal enzyme.

Adverse Reactions:

Sedation, Fatigue, Tinnitus, Diplopia, Euphoria, Epigastric Distrees, Hypotension and sense of tightness in chest.

Therapeutic uses:

  1. Suppression of Urticaria, pruritus, sneezing, rhinorrhoea and other allergic reactions.
  2. As sedative and hypnotic.
  3. For treatment of Parkinsonism.
  4. Motion sickness and nausea-vomiting of pregnancy.
  5. For the treatment of vertigo.
  6. Relief from the acute muscle dystonia.
  7. Used as antitussive.

Contra Indication:

Hypersensitivity, Epilepsy, Alcoholics.

Special Precautions:

Pregnancy, Lactation, Patient intending to Drive.

Recent Antihistaminics:

Most of the classical H1 receptor blocking drugs are highly lipid soluble and has marked CNS adverse effects, they also have anticholinergic effects, antiserotonergic and anti bradykinin effects.

A new class of pure H1 receptor competitive blockers, which poorly cross the blood brain barrier, is now available. They are the most specific H1 antagonists, without any sedative or other central effects. They do not cause sedation in therapeutic doses and are useful in various conditions. They have a relatively narrow therapeutic range and some of them can cause fatal ventricular arrhythmias. It is used for the treatment of allergic rhinitis, urticaria, and dermographia. E.g. Loratidine, Desloratidine, Terfenadine, Astemizole, Cetrizine, Levocetrizine.


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