Although, Antihistamines has been used widely, the pharmacokinetic data of such drug is not recorded as much it should. They have been measured with metabolites to greater or lesser scale. Interpretations of such data is with difficulty. Plasma concentration is taken after 2 to 3 hours for the oral administration of syrup or tablet form. Most drugs like Chlorpheniramine, Diphenhydramine, Promethazine are metabolised in the liver. The body clearance for adults is within the range of 5 to 12 ml/kg/min while the elimination is half the range from 3 hours to 18 days. It has large volume of distribution about 4L/kg. The elimination in children is half time shorter than in adults. Patients with alcohol related liver disease, it takes slower to eliminate. Whereas patients with chronic renal disease, Chlorpheniramine stays longer in the system. The older generation of H1 receptor antagonist like Brompheniramine, Chlorpheniramine and Hydroxyzine, its suggested dosage for an adult is once daily. Antihistamine can differ in the chemical structure and safety. With careful use the effects of Antihistamine can be reduced or avoided. Most of the H1 receptors antagonist are usually well absorbed in the body. The antagonist does not store tissues when the drug or medicine is given repeatedly.


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