Most adults are asked to fast about 8–12 hours before fasting blood work, but the exact time depends on the specific test and your doctor’s instructions. For many people, this works out to not eating after late evening and doing the test first thing in the morning so most of the fast happens while sleeping.

Typical fasting time

  • Many standard fasting blood tests (like fasting glucose and many lipid/cholesterol panels) use an 8–12 hour fasting window.
  • Some providers and educators recommend closer to 10–12 hours to be on the safe side for lipid and metabolic tests, unless your own clinician says otherwise.

Tests that often need fasting

  • Fasting glucose, some diabetes screening tests, and many cholesterol/lipid panels are commonly done after fasting.
  • Certain liver, kidney, iron, and vitamin tests may also require fasting, but this varies by lab and doctor, so specific instructions from your provider always come first.

What “fasting” usually means

  • No food and no drinks with calories (including juice, milk, sugary drinks, or coffee with cream/sugar) during the fasting window; plain water is typically allowed and encouraged unless your provider says otherwise.
  • Alcohol often needs to be avoided for a longer period before some tests, so check your written lab instructions if you have them.

When to ask your doctor

  • If your order includes multiple tests, ask whether all of them require fasting and exactly how many hours are needed so you do not over‑ or under‑fast.
  • If you took food, coffee, or medication by mistake during the fasting window, call the lab or your clinician; sometimes the test can still be done, but other times they may reschedule to keep results accurate.

Current general guidance: plan on 8–12 hours of fasting for “fasting” blood work and follow the specific timing your own doctor or lab gives you.