Losartan: Reduced levels of active metabolites w/ rifampicin & fluconazole. Increased serum K w/ K-sparing diuretics (eg, spironolactone, triamterene, amiloride), K supplements or salt substitutes containing K. Reduced excretion of lithium. Attenuation of antihypertensive effects w/ NSAIDs (ie, selective COX-2 inhibitors, acetylsalicylic acid at anti-inflammatory doses & non selective NSAIDs). Higher frequency of adverse events eg, hypotension, hyperkalaemia & decreased renal function (including acute renal failure) w/ ACE inhibitors. May increase risk of hypotension w/ TCAs, antipsychotics, baclofene, amifostine. Hydrochlorothiazide: Potentiation of orthostatic hypotension w/ alcohol, barbiturates, narcotics or antidepressants. Risk of lactic acidosis w/ metformin. Additive effect w/ other antihypertensives. Impaired absorption w/ cholestyramine & colestipol resins. Intensified electrolyte depletion, particularly hypokalemia w/ corticosteroids, ACTH; amphotericin B (parenteral), stimulant laxatives or glycyrrhizin (from liquorice). Possible decreased response to pressor amines (eg, adrenaline). Possible increased responsiveness to non-depolarizing skeletal muscle relaxants (eg, tubocurarine). Reduced renal clearance of lithium & increased risk of toxicity. May raise serum level of uric acid w/ uricosurics; may increase the incidence of hypersensitivity reactions to allopurinol. Increase bioavailability w/ anticholinergics (eg, atropine, biperiden). May reduced renal excretion of cytotoxics (eg, cyclophosphamide, methotrexate). May enhance toxic effects of salicylates. Isolated reports of haemolytic anaemia w/ methyldopa. May increase risk of hyperuricaemia & gout-type complications w/ cyclosporin. Serum K disturbances w/ digitalis glycosides & antiarrhythmics. Class Ia (eg, quinidine, hydroquinidine, disopyramide) & class III (eg, amiodarone, sotalol, dofetilide, ibutilide) antiarrythmics; some antipsychotics (eg, thioridazine, chlorpromazine, levomepromazine, trifluoperazine, cyamemazine, sulpiride, sultopride, amisulpride, tiapride, pimozide, haloperidol, droperidol); bepridil, cisapride, diphemanil, erythromycin IV, halofantrin, mizolastin, pentamidine, terfenadine, vincamine IV. May increase serum Ca levels w/ Ca salts & supplements. May interfere w/ tests for parathyroid function. Risk of symptomatic hyponatremia w/ carbamazepine. Increased risk of acute renal failure w/ iodine contrast media.