Antibiotics are drugs that kill bacteria. But they only work against bacteria. They don't kill viruses, so they won't work against a cold, the flu, or another viral illness. Unless you have a bacterial infection, it's best to avoid the possible harmful effects of antibiotics, which may include:

• Side effects . Antibiotics can cause nausea and diarrhea and can make you more sensitive to sunlight. Most of these common side effects are mild. But some side effects, such as allergic reactions, can be severe. They can cause shortness of breath or even death. If you have an unexpected reaction to an antibiotic, tell your doctor before you try another medicine.
• Other infections . Antibiotics kill most of the bacteria in your body that are sensitive to them, even the "good" bacteria that help your body. Antibiotics can ruin the balance of bacteria in your body, leading to an upset stomach, diarrhea, vaginal infections, or other problems.
• Bacterial resistance . Each time you take antibiotics, you are more likely to have some bacteria that the medicine doesn't kill. Over time, these bacteria change and become harder to kill. They become resistant to the medicine. The antibiotics that used to kill them no longer work.

If you and your doctor decide that you need an antibiotic, carefully follow the instructions for taking the medicine.

• Take the whole dose for as many days as your doctor tells you to, unless you have side effects you did not expect (in which case, call your doctor).
• Be sure you know any special instructions for taking the medicine. They should be printed on the label, but it’s also a good idea to check with your doctor and pharmacist.
• Keep antibiotics in a cool, dry place. Check the label to see if you should store them in the refrigerator.
• Never give an antibiotic prescribed for one person to someone else.
• Do not save any extra antibiotics. And do not take one prescribed for another illness unless your doctor tells you it is okay. Ask your pharmacist about how to safely throw away your leftover medicine.