Antihistamines

Allergies

An immune system response to harmless substances causing symptoms like sneezing, runny nose, and itchy eyes.

Histamine

A compound present in various body organs and cells, involved in allergic reactions.

Histamine Receptors

Proteins where histamine binds, influencing various physiological responses.

Antihistamines

Drugs that block histamine action at H1 receptors, reducing allergy symptoms.

How Antihistamines Work

Antihistamines bind to receptors, preventing histamine from causing allergic reactions.

First Generation Antihistamines

Older antihistamines like diphenhydramine and chlorpheniramine.

Second Generation Antihistamines

Modern antihistamines like cetirizine and loratadine with fewer side effects.

Structure Activity Relationships

Factors affecting the effectiveness of antihistamines based on their chemical structure.

Aminoalkyl Ether Derivatives

Chemical structure of antihistamines like diphenhydramine for H1 receptor activity.

Ethylenediamines

Antihistamines with a specific chemical structure for H1 receptor antagonism.

Tripelennamine

An antihistamine prepared by reacting 2 aminopyridine with β-dimethylaminoethyl chloride and benzyl bromide in the presence of sodamide, used to treat various allergic reactions.

Propylamine Derivatives

Antihistamines with a general structure possessing sp3 or sp2 carbon bridge between terminal tertiary amino group and diaryl moieties, exhibiting significant anticholinergic activity.

Pheniramine

An antihistamine available as pheniramine maleate, effective in treating allergic rhinitis, vasomotor rhinitis, and allergic conjunctivitis.

Chlorpheniramine

A Histamine H1-receptor antagonist used for symptomatic relief of hypersensitivity reactions, causing moderate sedation and having antimuscarinic activity.

Brompheniramine

An antihistamine with anticholinergic and sedative effects, effective for hay fever and upper respiratory allergic symptoms.

Triprolidine

A potent antihistamine used for hypersensitivity reactions, including urticaria, rhinitis, and conjunctivitis, also used for cough and common cold treatment.

Trimeprazine

A phenothiazine derivative effective for pruritic symptoms due to urticaria and various allergic and non-allergic conditions.

Promethazine

An antihistaminic agent with antimuscarinic and antiemetic properties, used for allergies, anxiety, and motion sickness.

Methdilazine

An antihistaminic agent used for allergic rhinitis, vasomotor rhinitis, allergic conjunctivitis, and anaphylaxis, possessing sedative effects and local anesthetic properties.

Cyclizine

A piperazine derivative acting as a histamine H1-receptor antagonist, used to control post-operative and drug-induced vomiting, and motion sickness.

Antazoline phosphate

White, bitter taste, crystalline powder occurring as a phosphate salt, soluble in water, used for rhinitis and conjunctivitis treatment.

Ebastine

Potent, long-acting antihistamine, piperidine derivative, acts against early and late phase allergic reactions without cardiovascular or CNS effects.

Carebastine

Active metabolite of ebastine, selective histamine H1 peripheral receptor antagonist without CNS or anticholinergic effects.

Second-generation antihistamines

Bind only to peripheral H1 receptors, reduce allergic response with minimal sedation, prolonged effects, divided into piperazine and piperidine derivatives.

Histamine H2-receptor blockers

Drugs blocking H2 receptors to inhibit gastric acid secretion, used for gastric ulcers, including cimetidine, ranitidine, and famotidine.

Cimetidine

H2-histamine receptor antagonist inhibiting gastric acid secretion, also inhibits cytochrome P450, used for ulcers.

Ranitidine

H2-histamine receptor antagonist more potent than cimetidine, fewer side effects, longer acting, used for ulcers.

Famotidine

Newer H2-histamine receptor antagonist more effective than ranitidine, less than 45% oral absorption, used for ulcers.

Side effects

Central nervous system reactions, gastrointestinal problems, hematologic reactions, and various other adverse effects associated with antihistamines.