Amoxicillin antibiotic pills in their blister pack in their box. Amoxicillin is a moderate spectrum antibiotic of the beta-lactam type, a derivative of penicillin. It works by preventing the formation of the cell walls of bacteria, which is fatal to them during cell division. However, some bacteria produce an enzyme, beta-lactamase, which destroys the drug before it can have this effect. Resistance to amoxicillin and other beta-lactam antibiotics is increasing, especially in hospitals and other environments where there is widespread use of antibiotics.