Sedating antihistamines effect is related to
In common use, the term "antihistamine" refers only to H In type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface Ig E antibodies on mast cells and basophils.Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil.Once released, the histamine can react with local or widespread tissues through histamine receptors.Histamine, acting on H-antihistamines help against these effects, they work only if taken before contact with the allergen.If an antihistamine fails to provide adequate relief, switch to a drug from a different chemical class.Individual drugs may be effective in no more than 40% of patients, and provide 50% relief of allergic symptoms. A large number of additional reactions have been reported.
Promethazine (Phenergan), in contrast, is a phenothiazine, chemically related to the major tranquilizers, and while it is used for treatment of allergies, may also be used as a sedative, the relieve anxiety prior to surgery, as an anti-nauseant, and for control of motion sickness.
systemic, topical or inhaled drugs that antagonize histamine and control allergies, e.g.
hayfever (seasonal rhinitis), drug allergies, urticaria (induced by insect bites); injected antihistamine (chlorphenamine) is used as an adjunct to adrenaline in the treatment of anaphylaxis and angioedema; modern antihistamines (e.g.
Monoamine oxidase inhibitor antidepressants (phenelzine [Nardil], tranylcypromine [Parnate]) may prolong and increase the effects of some antihistamines.
When used with promethazine (Phenergan) this may cause reduced blood pressure and involuntary movements.