Penicillin antibiotics are used to treat bacterial infections such as strep throat, pneumonia, and skin infections. They also have other uses, including treating urinary tract infections and some sexually transmitted infections (STIs). Penicillin antibiotics work by killing bacteria that's causing the infection. Examples such as amoxicillin, penicillin V potassium (Penicillin VK), and dicloxacillin are available as oral pills. Some penicillin antibiotics like Bicillin L-A (penicillin G benzathine) and ampicillin are injections. Your prescriber will choose the penicillin antibiotic that's most appropriate for treating your infection.