- Concomitant taking of the following drugs: Riluzole, Carbamazepine, Phenytoine, Primidone, Colestyramine, activated charcoal, cumarin, Cyclosporine, Methotrexate, Methoxyflurane, Theophylline, Phenobarbital; or drug classes: Antacids (containing Al, Mg, Ca), Retinoids, Digitalis Glycosides