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.