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.
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.
Several levels of evidence strongly indicate diphenhydramine (similar to chlorpheniramine) can block the delayed rectifier potassium channel and, as a consequence, prolong the QT interval, leading to cardiac arrhythmias such as torsades de pointes.