May increase exposure when is co-administration w/ sensitive substrates of P-gp (digoxin), BCRP (rosuvastatin) & OATP (pravastatin). Contraindicated w/ potent inducers of P-gp or of CYP2B6, CYP2C8, CYP3A4 (eg, rifampicin, rifabutin, St. John's wort, carbamazepine, phenobarb, phenytoin) as may lead to reduced therapeutic effect. Not recommended w/ moderate P-gp inducers or CYP inducers (eg, oxcarbazepine, modafinil, efavirenz) as may decrease sofosbuvir or velpatasvir plasma conc. May increase plasma conc w/ P-gp or BCRP inhibitor, or inhibitors of OATP, CYP2B6, CYP2C8 or CYP3A4. Liver function may change during treatment w/ vit K antagonists.