These drugs competitively antagonise actions of histamine at the H1 receptors, but there are quantitative differences, specially in the sdative property. They effectively block histamine induced bronchoconstriction, contraction of intestinal and other smooth muscle and triple response-specially wheal, flare and itch. Fall in produced by low doses of histamine is blocked. Release of Adr. from adrenal medulla in response to histamine is abolished. Constriction of larger blood vessel by histamine is also antagonised. These produce variable degree of C.N.S. depression. Indications Allergic conditions (hypersensitivity reactions) such as perineal and seasonal allergic rhinitis, urticaria, contact dermatitis, pruritus, insect bites, drug rashes, Meniere’s disease, motion sickness & other forms of nausea & vomiting. Dosage As shown against individual drugs. Contra-Indications Hypersensitivitiy, Pregnancy, nursing mothers, Prostatic hypertrophy and a tendency towards residual urine formation. Premature or new born infants; narrow angle glaucoma. Treatment of lower respiratory tract infections or acute asthmatic attacks. Epilepsy, concomitant therapy with MAO inhibitors, alcohol. Special Precautions Even at the dosages prescribed, these preparations may affect the patients alertness to such an extent that the ability to drive or operate machinery is impaired. The effect of drugs acting on the C.N.S. (e.g. tranquillizers, neuroleptic agents, sedatives and MAO inhibitors) and of alcohol may be potentiated. Side Effects G.I. disturbances, headache, dizziness, Dry mouth,blurred vision.Sedation through C.N.S. stimulation accompanied by hallucinations, convulsions.

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