Methapyrilene HCl was historically used as an antihistamine in over-the-counter sleep aids and cold medications due to its sedative effects. It worked by blocking histamine receptors, helping to relieve allergy symptoms and induce drowsiness. However, it has been discontinued in many countries because of safety concerns, particularly its association with liver toxicity and potential carcinogenicity in animal studies. As a result, it is no longer used in modern pharmaceutical formulations and has been replaced by safer alternatives.