3TC Drug Interactions

The likelihood of interactions is low due to limited metabolism and plasma protein-binding and almost complete renal elimination of unchanged lamivudine.
Lamivudine is predominantly eliminated by active organic cationic secretion. The possibility of interactions with other medicinal products administered concurrently should be considered, particularly when their main route of elimination is active renal secretion via the organic cationic transport system eg, trimethoprim. Other active substances (eg, ranitidine, cimetidine) are eliminated only in part by this mechanism and were shown not to interact with lamivudine.
Active substances shown to be predominately excreted either via the active organic anionic pathway or by glomerular filtration are unlikely to yield clinically significant interactions with lamivudine.
Zidovudine: A modest increase in C_max (28%) was observed for zidovudine when administered with lamivudine; however, overall exposure (AUC) was not significantly altered. Zidovudine had no effect on the pharmacokinetics of lamivudine (see Pharmacokinetics under Actions).
Trimethoprim/Sulfamethoxazole: Administration of trimethoprim/sulfamethoxazole 160 mg/800 mg (co-trimoxazole) causes a 40% increase in lamivudine exposure because of the trimethoprim component. However, unless the patient has renal impairment, no dosage adjustment of 3TC is necessary (see Dosage & Administration). Lamivudine has no effect on the pharmacokinetics of trimethoprim or sulfamethoxazole. The effect of co-administration of 3TC with higher doses of co-trimoxazole for the treatment of Pneumocystis carinii pneumonia and toxoplasmosis has not been studied.
Zalcitabine: Lamivudine may inhibit the intracellular phosphorylation of zalcitabine when the 2 medicinal products are used concurrently. 3TC is therefore not recommended to be used in combination with zalcitabine.