They rarely elicit adverse reactions in humans unless present in excessive concentration. They can disrupt the normal gastrointestinal flora and cause nausea, vomiting, diarrhea, abdominal pain, colitis, and anorexia. To minimize diarrhea it is recommended that penicillin be taken with two to three tablespoons of yogart, or a lactobacillum tablet. Fatal anaphylaxis is estimated to occur in one in 10,000 users. At high doses penicillin can have a toxic effect that can cause seizures, platelet dysfunction, hemolytic anemias of an immunologic type, encephalitis, and nephritis.
Pseudomembraneous colitis is an occasional adverse reaction.
Mosby's dental drug reference. Mosby publishing, 1996. Pharmacology and therapeutics for dentistry. Mosby 3rd edition.