Michael Smith, MD: Antibiotics are incredible life-saving drugs. They're great for bacterial infections, like strep throat and many ear infections. But they aren't cure-all wonder pills. Here are some things you should know before taking one. Antibiotics only treat bacterial infections. If you have a virus, they aren't going to help you one bit. So you never want to take an antibiotic just to be safe. Most common illnesses are viral. That means a cold, flu, coughs, most sore throats and sinus infections shouldn't be treated by antibiotics. You may be nervous when you see green mucus, but that's actually not a sign that you need an antibiotic. When your immune system fights off an infection, its perfectly normal for mucus to change color. Antibiotics don't come without risks. They can cause mild side effects like an upset stomach, to serious allergic reactions. Most frightening, overuse of antibiotics can create stronger bacteria or superbugs that can cause big problems, and are much harder to kill. Remember that if you don't need an antibiotic, that's a good thing. For WebMD, I'm Dr. Michael Smith.