Category B (‘Restrict’) products, also known as Highest Priority Critically Important Antibiotics, are only to be used as a last resort. These include quinolones (such as fluoroquinolones), 3rd and 4th generation cephalosporins, and polymyxins, including colistin. A new intermediate Category C (‘Caution’) has been created for antibiotics which should be used when there is no available product in Category D (‘Prudence’) that would be clinically effective.