THAT the simultaneous administration of penicillin by intermittent intramuscular injection over a period of eight days with daily intravenous injections of oxophenarsine hydrochloride in doses of 40 mg. is more effective thanpenicillin alone in the treatment of early syphilis of adults appears to be well substantiated. The therapeutic effects of the two compounds given simultaneously may be additive, but Eagle, Magnuson and Fleischman1 have reported that they were apparently synergistic in the treatment of acute syphilitic orchitis of rabbits. For example, these investigators found that when sodium penicillin was injected intramuscularly five times daily at four hour intervals for four days the total minimal curative dose was approximately 8,000 units per kilogram; the concurrent administration of oxophenarsine hydrochloride in subcurative doses of 0.4 mg. per kilogram daily for four days by intravenous injection, however, reduced the total minimal curative dose of penicillin to approximately 1,000 units per kilogram.
KOLMER JA, RULE AM. PENICILLIN IN THE TREATMENT OF EXPERIMENTAL SYPHILIS OF RABBITSII. The Synergistic or Additive Activity of Penicillin, Oxophenarsine Hydrochloride and Bismuth and Potassium Tartrate. Arch Derm Syphilol. 1947;56(2):179–186. doi:10.1001/archderm.1947.01520080039004