Decreased atazanavir concentration with tenofovir unless also co-administered with ritonavir. Increased serum concentration of tenofovir or co-administered drug if taken with drugs that are eliminated by active tubular secretion. Potentially Fatal: Increased risk of renal impairment with recent or concurrent use of nephrotoxic agents (e.g. aminoglycosides, amphotericin B, foscarnet, ganciclovir, pentamidine, vancomycin, cidofovir or interleukin-2); monitor renal function wkly if unavoidable. Increased didanosine levels and thereby increasing risk of pancreatitis and peripheral neuropathy, with a high treatment failure rate with concurrent use; avoid concurrent use.