Sedating antihistamines mechanism of action
Antihistamines are a class of agents that block histamine release from histamine-1 receptors and are used to treat the symptoms of an allergic reaction, such as edema (swelling), itch, inflammation (redness), sneezing, or a runny nose or watery eyes.
Antihistamines can be further divided into those unlikely to cause drowsiness (non-sedating antihistamines) or those likely to cause drowsiness (sedating antihistamines).
Antihistamines are used in the treatment of allergic reactions, colds, hay fever, hives, and insect bites and stings.
Some antihistamines may also be helpful in reducing anxiety, inducing sleep, or at preventing or treating motion sickness.
Certain H1 antagonists, such as promethazine, have a local anesthetic effect.
Other H1-antihistamines having an important antimuscarinic activity like diphenhydramine and dimenhydrinate, are used in preventive and curative treatment of motion sickness, but scopolamine which has no antihistamine effect seems more effective than them in preventive treatment.
Patients with severe hepatic (liver) disorders may need to start with a lower dose.The drugs of the first antihistamine generation are: promethazine, alimemazine, dexchlorpheniramine, brompheniramine, buclizine, carbinoxamine and doxylamine, are sedating and elicit a drowsiness which can be awkward.The first-genaration antihistamines have alpha adrenolytic activity which can decrease the vasoconstrictive effect of adrenaline and noradrenaline and an antimuscarinic effect with the corresponding adverse effects.Substances that act as inhibitors of the CYP3A4 enzyme such as ketoconazole, erythromycin, cimetidine, and furanocoumarin derivatives (found in grapefruit) lead to increased plasma levels of loratadine.