Effect of magnesium trisilicate on nitrofurantoin absorption

In vitro adsorption studies revealed that for an identical initial concentration of nitrofurantoin, magnesium trisilicate exhibited the greatest adsorptive capacity with bismuth oxycarbonate, talc, kaolin, and magnesium oxide exhibiting intermediate adsorptive powers, while aluminum hydroxide and calcium carbonate exhibited low or no adsorption properties. Trials to elute the drug with acidic or alkaline solution were unsuccessful. The in vivo absorption characteristics of nitrofurantoin and nitrofurantoin-magnesium trisilicate combination were evaluated in 6 healthy males. Administration of magnesium trisilicate with nitrofurantoin reduced the rate and extent of its excretion reflecting decrease in both rate and extent of absorption. The time during which the drug concentration in the urine was above the minimum effective concentration of 32 microgram/ml was also significantly reduced after administration of the antacid.