Antacids, iron, aluminum, calcium, magnesium, zinc salts reduce absorption. Concurrent use may cause increased levels of lithium, digoxin, halofantrine and theophylline; decreased concentrations of atovaquone. Increased risk of ergotism with ergot alkaloids. May cause failure of oral contraception. Potentially Fatal: Interferes with anticoagulant control. Nephrotoxic effects exacerbated by diuretics, methoxyflurane or other nephrotoxic drugs; avoid concurrent use with potentially hepatotoxic drugs. Increased incidence of benign intracranial hypertension with retinoids.