Interaction studies have shown that the potential for metabolic drug interactions is low.
Lamivudine is predominantly eliminated by active organic cationic secretion. The potential for interactions with concurrently administered drugs sharing this pathway as a major elimination route (eg, trimethoprim) should be considered, particularly if the patient has renal impairment. Other drugs (eg, ranitidine, cimetidine) are eliminated only in part by this mechanism and were shown not to interact with lamivudine.
Drugs shown to be predominantly excreted either via the active organic anionic pathway, or by glomerular filtration (eg, ganciclovir, foscarnet) are unlikely to yield clinically significant interactions with lamivudine.
Administration of trimethoprim, a constituent of co-trimoxazole, causes an increase in lamivudine exposure of about 40%. Lamivudine has a high therapeutic ratio and this increase is not considered clinically relevant unless the patient has renal impairment. Lamivudine has no effect on the pharmacokinetics of co-trimoxazole.
There is no known adverse interaction with α-interferon. There are no observed clinically significant adverse interactions in patients taking 3TC-HBV concurrently with commonly used immunosuppressant drugs (eg, cyclosporin A). However, formal interaction studies have not been performed.