My doctor has prescribed metronidazole for me to treat a bacterial infection. He emphasized I should continue taking the medication until it's all gone, even if I begin to feel better. I stopped taking it the last day (only 3 pills). My infection came back. Could that have happened because of the one day of missing the medication? Now I can't seem to get rid of the infection no matter how many times my doctor prescribes metronidazole. Can you become immune to an antibiotic?

-- Janice

There is a reason we always tell you to finish ALL medication prescribed, even if your symptoms improve. In fact, there are two reasons: First, if you stop too soon, the infection may return; second, the bacteria may develop resistance to the antibiotic. Overprescribing of antibiotics by doctors, and their misuse by patients (including not finishing the prescribed course), have led to an epidemic of resistant organisms in the world. A particularly nasty strain of staph is out there now that is resistant to all but one antibiotic; if it develops resistance to that one remaining weapon, we're all in trouble!

In your case, your doctor should perform a vaginal culture and run antibiotic sensitivity tests to see just what you have and what it is sensitive to. You may need a prolonged (several-week-long) course of high-dose metronidazole or another antibiotic altogether. And this time, follow the doctor's instructions EXACTLY -- for your sake, as well as the rest of the planet's!