Antibiotics come in a variety of chemical classes, but in three functional classes:

1. Antibiotics which target cell-wall synthesis. Because bacteria have cell walls and animal cells do not, this class of antibiotics tends to have the greatest efficacy with the least amount of side effects. The ß-lactams, like penicillin and amoxicillin are examples, as are the cephalosporins like cephalexin.

1. Antibiotics which target DNA or RNA synthesis. These antibiotics inhibit the function of DNA/RNA polymerases, or accessory proteins such as topoisomerases, and thereby inhibit either DNA replication or RNA synthesis. Fluoroquinolines such as ciprofloxacin are examples of DNA synthesis inhibitors, rifampicin is an example of an RNA synthesis inhibitor.

1. Antibiotics which target protein synthesis. These antibiotics usually target bacterial ribosomes and arrest the synthesis of new proteins. They are often bacteriostatic (keep bacteria from growing), rather than bacteriocidal (kill bacteria). Streptomycin is an example.

Some newer classes of antibiotics, such as those which target lipid synthesis, are in development, but they have not yet reached the clinic.