Antibiotics are medicines that kill or stop the growth of bacteria (or sometimes parasites). They work in different ways, but most of them interfere with the bacterium’s ability to make or use its cell wall. This usually stops the infection from getting worse. Some antibiotics also work by attacking the bacterial cells directly. This usually makes the bacteria die, although it can also leave harmful substances behind in the body that may cause side effects.

Many infections caused by bacteria can be treated successfully without antibiotics. The body’s natural defences can clear most ear, nose and throat infections, and most viral illnesses (such as the common cold).

Bacteria can become resistant to antibiotics, so it is important to take them exactly as prescribed. This will reduce the chance of side effects, and help prevent bacteria from becoming resistant to the medicines. In the United States, you need a prescription from a doctor to get most oral antibiotics. This helps ensure that the medicine is correctly selected for your infection, and that it is safe for you based on your medical history.

When choosing an antibiotic, doctors try to predict which ones will be most effective against the specific bacteria that are causing the infection. If the infection is mild and a single type of bacteria seems likely to be responsible, this can be done quickly by testing samples of blood, urine or tissue for susceptibility to various antibiotics. If the infection is more serious, the infecting bacteria must be identified by laboratory tests, which can take a day or two to complete.