Influence of viscosity on absorption from nitrofurantoin suspensions.

Nitrofurantoin, 200 mg, was administered orally to 11 subjects in an aqueous reference dispersion and in five suspensions having the same rheogram. Algin, carbomer, guar gum, methylcellulose, and colloidal magnesium aluminum silicate were the five suspending agents employed. Complexation was demonstrated by dialysis between nitrofurantoin and methylcellulose and between nitrofurantoin and carbomer; however, physiological availability was not altered by the interaction. The viscosity increase slowed absorption and urinary excretion, thus delaying the time of the maximum excretion rate without a decrease in bioavailability. A clinically acceptable urinary nitrofurantoin concentration was maintained for at least 2 hr longer by a viscosity increase.