Penicillin is relatively safe, although a small percentage of people are allergic to the antibiotic. In people who are not allergic, penicillin still carries a small risk of side effects, such as upset stomach, nausea, vomiting and diarrhea, and a flat, red rash that resolves on its own, according to the medical resources UpToDate and StatPearls.
Specifically, penicillin G can cause electrolyte imbalances, particularly when given in large doses, and may cause effects like muscle spasms and pain, fever or low blood pressure.