Erythromycin is an antibiotic used to treat various bacterial infections. It works by inhibiting the growth of bacteria, making it effective against respiratory tract infections, skin infections, and some sexually transmitted diseases. Erythromycin is often prescribed for patients who are allergic to penicillin. This medication can be taken orally in tablet or liquid form, or it can be administered intravenously in a hospital setting. Common side effects may include stomach upset, diarrhea, and nausea. It's important to complete the full course of treatment to prevent antibiotic resistance.