Quinidine Magnesium hydroxide

Drugs that have been associated with elevations in quinidine concentrations include acetazolamide, the antacids magnesium hydroxide and calcium carbonate, and the H2-receptor antagonist cimetidine. Cimetidine inhibits the hepatic metabolism of quinidine. Phenytoin, rifampin, and barbiturates increase the hepatic metabolism of quinidine and reduce its plasma concentrations. 

Large rises in urinary pH due to the concurrent use of some antacids, diuretics or alkaline salts can cause quinidine retention, which could lead to quinidine toxicity, but there seems to be only one case on record of an adverse interaction (with an alumini-um/magnesium hydroxide antacid). Aluminium hydroxide alone appears not to interact. 

A patient on quinidine who took eight Afy/ow/a tablets daily (aluminium hydroxide gel 200 mg, magnesium hydroxide 200 mg and simeticone 20 mg) for a week and a little over 1 litre of fruit juice (orange and grapefruit) each day developed a threefold increase in serum quinidine levels (from 8 to 25 mg/L) and toxicity. However, note that the grapefruit juice , (p.280) may have contributed. In 6 healthy subjects, this dose oiMylanta for 3 days produced consistently alkaline urine in 4 subjects, and in 5 subjects when combined with fruit juice. ... 

Remon JP, Belpaire E, Van-Severen R, Braeckman P. Interaction of antacids with anti-arrhythmics. Part 5. Effect of aluminum hydroxide and magnesium oxide on the bioavailability of quinidine, procainamide, and propranolol in dogs. Arzneimittel Forschung 1983 33(1) 117-120. 

---