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Introduction of Antihistamines

Histamines are released by the body by certain mast cells when exposed to allergens. The release of histamine produces symptoms such as itching, sneezing, hay fever, and nasal irritation. In some people it can even close up the bronchial tube making breathing difficult. Antihistamines work by replacing the histamines thereby combating their effect.

Types of Antihistamines

Antihistamines include a broad class of pharmacological agents. The types of antihistamines are:

  1. The first-generation antihistamine like diphenhydramine (H1-receptor antagonists). These are is capable of producing significant CNS effects.
  2. The newer, second-generation antihistamine. These are ‘nonsedating’ H1 blockers like loratadine. Cimetidine (H2-receptor antagonists). These primarily work on gastric mucosa and hinder gastric secretion.
  3. Experimental antihistamines that act on presynaptic (H3 receptors).
  4. The recently discovered H4 receptors.

Therapeutic effects of antihistamines

Antihistamines are commonly used under the following circumstances:
  • To provide relief from the symptoms of hay fever like sneezing, itching of the eyes, nose and throat, and rhinorrhoea (runny nose).
  • To provide relief from rhinitis and conjunctivitis (inflammation of the nose and eyes).
  • Some antihistamines are also used in the treatment of chronic hive-like rash such as urticaria and generalised pruritus (itch). It reduces the severity of the rash and associated itching.
  • Antihistamines prevent motion sickness and other causes of nausea.
  • Antihistamines stimulate appetite in adults and children.
  • Cetirizine, an antihistamin is used in addition to asthma medicine to prevent as well as reduce breathing difficulty.
  • Diphenhydramine, an antihistamin decreases stiffness and tremors in patients with Parkinson’s disease.
  • Cyproheptadine, another antihistamin treats vascular headache.      
  • Some antihistamin treats nervous and emotional conditions to help control anxiety. It also controls anxiety and produces sleep before surgery. Their sedating and anti-sickness effects is used in the care of the terminally ill patients.
  • Antihistamines are used in the emergency treatment of severe allergic reactions.
  • Antihistamines belonging to Cimetidine class suppress histamine-induced gastric secretion and are used in treating gastric and duodenal ulcers.


Side effects of Antihistamines

When used in sufficiently large doses, nearly all antihistamines produce undesirable side effects. However, the incidence and severity of the side effects depend both on the patient and on the properties of the specific drug. Antihistamines can cause unwanted side effects in individuals with narrow glaucoma, and breast-feeding mothers.

Side effects of anti histamines are as follows.


  • Drowsiness
  • Dry mouth
  • Urine retention
  • Headache
  • Blurred vision
  • Gastrointestinal irritation


  • Seizures
  • Hallucinations (seeing, hearing or feeling things that are not there)
  • Mental depression
  • Short-acting antihistamines have been shown to impair cognition (slow learning) in children (even in the absence of drowsiness), which is why they are generally not recommended in children.


  • Fast or irregular heartbeat, and
  • Urine retention in males.


Antihistamines are available in different dosage forms:

Antihistamines come in several forms like: 

  • Syrup,
  • Tablet,
  • Capsule,
  • Solution,
  • Tablet,
  • Chewable,
  • Powder for Suspension,
  • Liquid,
  • Disintegrating capsule,
  • Liquid Filled,
  • Elixir,
  • Film,
  • Suspension, and
  • Capsule.

Drug Interactions of Antihistamines

Drugs that cause drowsiness, such as alcohol, sleep preparations, sedatives, or tranquilizers should be avoided while on antihistamines. Intake of MAOIs like isocarboxazid (Marplan), phenelzine sulfate (Nardil), or tranylcypromine (Parnate) should be avoided within 14 days of antihistamines.

Antihistamines are often combined with various decongestants and/or pain relievers. If any of these combination medicines are taken, it’s important to understand each of the active ingredients and the interactions they may have with other medicines.

For drug interactions specific to a particular antihistamine, a doctor or pharmacist should be consulted.

Related topics:

Allergies and Seasonal allergies

Written by: Healthplus24 team
Date last updated: June 30, 2011

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