Ugh, YES, bad breath is one of the side effects, and it's awful- I am on my ninth day of a water-fast and I feel like it is only getting worse. When you fast waste passes through the lungs, which is an eliminative organ. Toxic buildup increases the bacteria between your teeth and your tongue becomes coated with waste. (Gross.) Brushing the tongue with a toothbrush, using dental floss, and washing with mouthwash will reduce bad breath.