The stability of procainamide hydrochloride 5, 50, and 100 mg/mL in an extemporaneous oral liquid formulation was studied. Oral liquids containing procainamide hydrochloride 5, 50, or 100 mg/mL were compounded from procainamide hydrochloride capsules, sterile water for irrigation, and cherry syrup or a 70:30 mixture of 1% methylcellulose and cherry syrup. The pH values of the liquids were 6 (unadjusted) and 5 (adjusted). The liquids were stored at 4-6 degrees C or 24-25 degrees C and tested for drug concentration by stability-indicating high-performance liquid chromatography at 0, 7, 14, 21, 28, 34, 147, and 180 days. The relationships among the variables drug concentration, type of vehicle, pH, and temperature and their effects on drug stability were analyzed by using repeated-measures analysis of variance. With respect to effects on stability across time, there was a significant three-way interaction among drug concentration, type of vehicle, and temperature. The effect of concentration on stability seemed to depend on the vehicle used, but this dependence was different for different temperatures and different pH values. When trends over time were considered, a significant interaction among time, vehicle type, and temperature was found. This reflected the stability of the concentration of procainamide hydrochloride over time under refrigeration for both vehicles, compared with the steady decline in drug concentration at room temperature. Overall, drug concentration was fairly stable over time in the 5- and 50-mg/mL oral liquids but declined steadily in the 100-mg/mL liquid.(ABSTRACT TRUNCATED AT 250 WORDS)