Stability of Penicillin G in Oleaginous Formulations Containing Colloidal Silica

Stability or procaine and potassium salts of penicillin G in oleaginous Materials containing colloidal silica was studisd. The instability of penicillin G in these bases seeMed to be due pr!Marily to the surface acidity or the silica in conjunction with the ester constituents of oils. The stability or the peanut oil/silica/ penicillin G preparations could be !Mproved by reducing the surface acidity of the silica by esteritication. Dehydration of the silica also gave fol'lllUlations with !Mproved stability, but with unacceptable rheological properties. 'l'he color reactions of procaine penicillin G/silica/ oleaginous Materials appeared to be unrelated to the stability of the drug and Might be due to interactions of procaine with traces of aldehydes present in the oily Materials.