Most antibiotics have two names, the trade or brand name, created by the drug company that manufactures the drug, and a generic name, based on the antibiotic's chemical structure or chemical class. Trade names such as Keflex and Zithromax are capitalized. Generics such as cephalexin and azithromycin are not capitalized. Each antibiotic is effective only for certain types of infections, and your doctor is best able to compare your needs with the available medicines. Also, a person may have allergies that eliminate a class of antibiotic from consideration, such as a penicillin allergy preventing your doctor from prescribing amoxicillin. In most cases of antibiotic use, a doctor must choose an antibiotic based on the most likely cause of the infection. For example, if you have an earache, the doctor knows what kinds of bacteria cause most ear infections. He or she will choose the antibiotic that best combats those kinds of bacteria. In another example, a few bacteria cause most pneumonias in previously healthy people. If you are diagnosed with pneumonia, the doctor will choose an antibiotic that will kill these bacteria.