Stability of phenylbutazone in presence of pharmaceutical colors

The degradation of phenylbutazone was studied in the presence of lakes suitable for coloring the sugar coats of phenylbutazone tablets. The drug was degraded, in light, in the presence of erythrosine sodium. The degradation probably proceeds via singlet oxygen generated by the light-exicited dye. The degradation may be important in some quality control procedures and can lead, for example, to unusual results in dissolution rate testing.