Catecholamine - depleting drugs such as reserpine, may cause additive effects and precipitate marked hypotension or bradycardia; betablockers may increase patients' response to allergens and reduce the effectiveness of usual doses of adrenaline in treating allergic reactions; increased levels of metoprolol with artemether/lumefantrine (manufacturer advises to avoid); possible severe hypotension and heart failure with nifedipine, nisoldipine, verapamil; risk of severe hypertension with adrenaline, noradrenaline (norephedrine); increased risk of bradycardia, AV block and myocardial depression with amiodarone (potential risk may last several weeks due to amiodarone's long half life), antiarrhythmics (including cardiac glycosides), diltiazem, flecainide, mefloquine (bradycardia); hydrochlorothiazide may reduce the excretion of lithium, increasing the risk of lithium toxicity; hypokalaemia potentially caused by metoprolol and/or hydrochlorothiazide may increase risk of ventricular arrhythmias with amisulpride, pimozide, atomoxetine, sertindole, cardiac glycosides, disopyramide, flecainide, sotalol; increased risk of nephrotoxicity and ototoxicity with platinum compounds and aminoglycosides.

Effects of tubocurarine may be prolonged while corticosteroids may increase the risk of hypokalaemia; may precipitate azotemia in renal patients; increased hypotensive effects with: alcohol, a-blockers, general anaesthetics, hydralazine, levodopa, MAOIs, methyldopa, nitrates, phenothiazines, ACE inhibitors, adrenergic neurone blockers, aldesleukin, alprostadil, antiotensin-II receptor antagonists, TCAs, anxiolytics and hypnotics, baclofen, calcium-channel blockers, clonidine, diazoxide, minoxidil, moxisylyte, moxonidine, sodium nitroprusside, tizanidine; hydrochlorothiazide increases plasma concentration of fluconazole; b-blockers may mask warning signs of hypoglycaemia with antidiabetics, increas