Losartan: Reduced levels of active metabolite w/ rifampin & fluconazole. Increased serum K w/ K-sparing diuretics (eg, spironolactone, triamterene, amiloride), K supplements, K-containing salt substitutes or other drugs increasing serum K (trimethoprim-containing products). Reduced lithium excretion. Attenuated antihypertensive effects w/ NSAIDs including selective COX-2 inhibitors. Avoid use w/ aliskiren. Other drugs affecting renin-angiotensin-aldosterone system. Amlodipine: Increased plasma conc w/ strong CYP3A4 inhibitors eg, ketoconazole, itraconazole, ritonavir. Lowered active metabolite conc w/ grapefruit juice.