Post-nasal drip may sound serious, but it's actually a natural bodily function. Post-nasal drip is mucus moving from your nose down the back of your throat. However, when a post-nasal drip becomes uncomfortable, it may be a sign of a sinus infection, cold or other illness. You may also realize that your breath smells worse than usual. However, post-nasal drip may not be responsible for your bad breath. Learn more about post-nasal drip and its effects on your health and tips for preventing it.