Antibiotics are powerful medicines that fight bacterial infections. Used properly, antibiotics can save lives. They either kill bacteria or keep them from reproducing. Your body's natural defenses can usually take it from there.

Antibiotics do not fight infections caused by viruses, such as

• Colds
• Flu
• Most coughs and bronchitis
• Sore throats, unless caused by strep

If a virus is making you sick, taking antibiotics may do more harm than good. Each time you take antibiotics, you increase the chances that bacteria in your body will be able to resist them. Later, you could get or spread an infection that those antibiotics cannot cure.

When you take antibiotics, follow the directions carefully. It is important to finish your medicine even if you feel better. Do not save antibiotics for later or use someone else's prescription.

Selecting an Antibiotic

Each antibiotic is effective only against certain bacteria. In selecting an antibiotic to treat a person with an infection, a doctor makes a best guess as to which bacterium is responsible. For some infections, doctors know that only certain types of bacteria may be responsible. If there is one antibiotic that is predictably effective against all of these bacteria, further testing is not needed. For infections that may be caused by many different types of bacteria or by bacteria whose susceptibility to antibiotics is not predictable, a laboratory will be asked to identify the infecting bacterium from samples of blood, urine, or tissue taken from the person. The infecting bacterium is then tested for susceptibility to a variety of antibiotics. These tests generally take a day or two to yield results and thus cannot guide the initial choice of which antibiotic is given.

Antibiotics that are effective in the laboratory do not necessarily work in an infected person, however. The effectiveness of the treatment depends on how well the drug is absorbed into the bloodstream, how much of the drug reaches the sites of infection in the body, and how quickly the body eliminates the drug. In selecting which antibiotic to use, a doctor also considers the nature and seriousness of the infection, the drug's possible side effects, the possibility of allergies or other serious reactions to the drug, and the cost of the drug.

Combinations of antibiotics are sometimes needed to treat severe infections, particularly in the first days when the bacterium's sensitivity to antibiotics is not known. Combinations are also important for certain infections in which the bacterium rapidly develops resistance to a single antibiotic. Infections caused by more than one bacterium, in which each bacterium is susceptible to a different antibiotic, are also treated with a combination of antibiotics.