Amoxicillin molecule. Computer artwork showing the molecular structure of amoxicillin. Amoxicillin is a beta-lactam antibiotic used for the treatment of a number of bacterial infections. It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. Atoms are represented as spheres: carbon (light-blue), hydrogen (white), nitrogen (dark-blue), oxygen (red), sulphur (yellow).