Antibiotic powder. Bottle of Erythroped antibiotic powder for adding to water to drink. This contains the drug erythromycin ethylsuccinate. This is a macrolide antibiotic which works by preventing the bacteria from producing proteins they need to survive. Erythromycin is often given to patients who are allergic to penicillin drugs, as it kills many of the same bacteria as they do. It is commonly used to treat syphilis, respiratory tract infections and pneumonia. Side-effects may include nausea, vomiting and diarrhoea.