Penicillin/ˌpenɪˈsɪlɪn/ Definition 1 of 3Pro Tip 1/3Medical Treatment Used specifically for treating certain bacterial infections, not viruses. She was given penicillin for her bacterial infection, but not for her flu. Pro Tip 2/3Allergy Warning Some people are allergic to penicillin, which is critical for safe usage. He wears a bracelet that alerts medics about his penicillin allergy. Pro Tip 3/3Not for Colds Avoid using 'penicillin' for common colds; it's ineffective against viral infections. Don't ask for penicillin for a cold; it won't help.