For a blood test that requires fasting, you typically need to fast for 8 to 12 hours before the test. During this fast, you should not eat or drink anything except water. Some tests, such as those for lipids, triglycerides, or cholesterol, may require fasting up to 12 to 14 hours. It is important to follow your doctor’s specific instructions, as fasting times can vary by test. You can usually eat again immediately after your blood is drawn.