Indications: Procaine penicillin is used to treat many different types of infections caused by bacteria, including syphilis. Contra-indications: A previous hypersensitivity reaction to any penicillin is a contraindication. Adverse Effects: Allergic Reactions Penicillin is a substance of low toxicity but does possess a significant index of sensitization. The following hypersensitivity reactions associated with use of penicillin have been reported: Skin rashes, ranging from maculopapular eruptions to exfoliative dermatitis; urticaria; serum-sicknesslike reactions, including chills, fever, edema, arthralgia, and prostration. Severe and often fatal anaphylaxis has been reported . As with other treatments for syphilis, the Jarisch-Herxheimer reaction has been reported. Procaine toxicity manifestations and hypersensitivity reactions have been reported. Gastrointestinal Pseudomembranous colitis has been reported with the use of penicillin G. Onset of pseudomembranous colitis symptoms may occur during or after antibiotic treatment. Drug Interactions: Tetracycline, a bacteriostatic antibiotic, may antagonize the bactericidal effect of penicillin and concurrent use of these drugs should be avoided. Concurrent administration of penicillin and probenecid increases and prolongs serum penicillin levels by decreasing the apparent volume of distribution and slowing the rate of excretion by competitively inhibiting renal tubular secretion of penicillin.