Only when the specific test requires it - fasting glucose and lipid panels are the classics, usually 8 to 12 hours with plain water allowed.
MedlinePlus notes fasting also rules out gum, smoking, and exercise. Most other common blood tests need no fasting at all, so ask when the test is ordered rather than guessing. Book the earliest morning slot and eat a normal dinner.
What to do
Common questions
Do I need to fast before blood work?
Only when the specific test requires it - and your order or provider's instructions will say so. Per MedlinePlus, fasting means nothing to eat or drink except plain water for the hours before the draw, usually 8 to 12 hours, and it also rules out gum, smoking, and exercise. The tests that typically require it: blood glucose tests (food moves blood sugar directly, so a fed number is not diagnostic) and lipid panels for cholesterol in many cases. Most other common tests - a CBC, thyroid panels, many hormone and vitamin levels - need no fasting at all, which is why asking beats guessing. The practical routine for a fasting draw: book the earliest morning slot, eat a normal dinner, bring a snack for afterward, and keep taking essential medications with water unless your provider said otherwise - ask about your specific medications when the test is ordered, not the morning of. Pymander, a free AI doctor available 24/7 by text, can help you sort your specific orders.