Valsartan: K supplements, K-sparing diuretics, salt substitutes containing K or other drugs altering K levels (eg, heparin). Attenuation of antihypertensive effect w/ NSAIDs. Increased systemic exposure w/ inhibitors of uptake transporter OATP1B1 (rifampin, ciclosporin) or efflux transporter MRP2 (ritonavir). Hydrochlorothiazide: Reversible increases in serum lithium & toxicity. Potentiates action of other antihypertensives (eg, guanethidine, methyldopa, β-blockers, vasodilators, Ca channel blockers, ACE inhibitors, angiotensin receptor blockers & direct renin inhibitors) & skeletal muscle relaxants (eg, curare derivatives). Increased hypokalemic effect w/ kaliuretic diuretics, corticosteroids, ACTH, amphotericin, carbenoxolone, penicillin G, salicylic acid derivatives or antiarrhythmics. Hyponatremic effect may be intensified w/ antidepressants, antipsychotics, antiepileptics. May alter glucose tolerance w/ insulin & oral antidiabetic agents. Risk of thiazide-induced hypokalemia or hypomagnesemia w/ digitalis glycosides. Diuretic & antihypertensive activity may be weakened w/ NSAIDs (eg, salicylic acid derivative, indomethacin) & selective COX-2 inhibitors. May increase incidence of hypersensitivity reactions to allopurinol & risk of adverse effects caused by amantadine; reduce renal excretion of cytotoxic agents & enhance their myelosuppressive effects eg, cyclophosphamide, methotrexate. Bioavailability may be increased by anticholinergics (eg, atropine, biperiden) or decreased by prokinetic drugs (eg, cisapride). Decreased absorption by cholestyramine or colestipol. Serum Ca elevation may be potentiated w/ vit D or Ca salts. Increased risk of hyperuricemia & gout-type complications w/ ciclosporin. Risk of hypercalcemia w/ Ca salts; hemolytic anemia w/ methyldopa. May enhance hyperglycemic effect of diazoxide. Orthostatic hypotension may be potentiated w/ alcohol, barbiturates or narcotics. May reduce response to pressor amines (eg, noradrenaline).