Anthralin stability in various vehicles.

The stability of anthralin in white soft paraffin, Lassar's paste, and Unguentum Merck was assessed using a sensitive assay for degradation products, over a 1-year period. The stability was found to vary with concentration, weaker preparations being less stable. Significant degradation was observed in Unguentum Merck, and this increased when salicylic acid was present. Preparations with Unguentum Merck could be stabilized with ascorbic or oxalic acid. Salicylic acid stabilized anthralin in Lassar's paste but had a negligible effect in white soft paraffin. Preparations with white soft paraffin were relatively stable.