Typically, you need to fast for 8 to 12 hours before having blood work done. During this fasting period, you should not eat or drink anything except plain water. The exact fasting duration may depend on the specific blood test, so it is important to follow your doctor's instructions. Drinking water is allowed and encouraged to stay hydrated, but you should avoid other liquids, smoking, chewing gum, and exercise during the fast. Scheduling your blood work in the morning is recommended so most of the fast happens while you sleep.