ZONALON Cream Ref.[50741] Active ingredients: Doxepin

Although doxepin HCl does have H₁ and H₂ histamine receptor blocking actions, the exact mechanism by which doxepin exerts its antipruritic effect is unknown. Zonalon Cream can produce drowsiness which may reduce awareness, including awareness of pruritic symptoms. In 19 pruritic eczema patients treated with Zonalon Cream, plasma doxepin concentrations ranged from nondetectable to 47 ng/mL from percutaneous absorption. Plasma levels from topical application of Zonalon Cream can result in CNS and other systemic side effects.

Once absorbed into the systemic circulation, doxepin undergoes hepatic metabolism that results in conversion to pharmacologically-active desmethyldoxepin. Further glucuronidation results in urinary excretion of the parent drug and its metabolites. Desmethyldoxepin has a half-life that ranges from 28 to 52 hours and is not affected by multiple dosing. Plasma levels of both doxepin and desmethyldoxepin are highly variable and are poorly correlated with dosage. Wide distribution occurs in body tissues including lungs, heart, brain, and liver. Renal disease, genetic factors, age, and other medications affect the metabolism and subsequent elimination of doxepin. (See PRECAUTIONS – Drug Interactions.)