PHARMACOLOGY

 PHARMACOLOGICAL ACTIONS ::

    General antihistamimic actions ::

All antihistamines block histamine induced vasodilatation allergy anaphylaxis triple response contraction of smooth muscles hypotension headache etc 

Central nervous system ::

Most of the antihistamines produce CNS depression of varying degree it may be sedation drowsiness or even sleep Children in particular are more sensitive to this effect Promethazine and diphenhydramine are more potent as sedatives Sedation is often accompanied by inability to concentrate dizziness and disturbances of co-ordination 

        Some antihistamines like phenindamine produce CNS stimulation leading to restlessness tremors and insomnia 

    Promethazine diphenhydramine and piperazine are also effective in motion sickness and some antihistamines are useful in parkinsonism Astemizole terfenadine and cetrizine are devoid of central depressant effects 

Local anaesthetic action ::

A large number of antihistamines such as promethazine mepyramine etc produce local anaesthesia

 

Miscellaneous actions ::

Diphenhydramine potentiates the actions of adrenaline some antihistamines produce atropine like actions such as dryness of mouth Some antihistamines like antazoline and triplenamine may produce quinidine like action when given through intravenous route