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Fasting for a Blood Test
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Fasting for a Blood Test

Fasting for a blood test requires avoiding all food and drink, except plain water, for several hours to ensure the accuracy of your results.

What is fasting for a blood test?

If your health care provider has told you to fast before a blood test, it means you should not eat or drink anything, except plain water, for several hours before your test. While you're fasting for a blood test, you should also not:

  • Chew gum
  • Smoke
  • Exercise

Your provider will give you a copy of instructions to follow if they ask you to fast for a test. If you have any questions not covered in those instructions, ask your provider at least a full day before the test so that you'll have time to properly prepare.

How long do I have to fast before the test?

You usually need to fast for 8 to 12 hours before a test. Your provider will tell you exactly how long to fast, depending on what test they've ordered. For certain tests, there may also be additional instructions. For example, you may be asked not to drink alcohol for a longer period of time.

If your blood test requires fasting, you can usually schedule the test early in the morning. That way, most of your fasting time will be while you are sleeping.

Why do I need to fast before my blood test?

When you eat and drink, your bloodstream absorbs sugar, fat, protein, and other nutrients in those foods and beverages. These nutrients can then change the levels of various substances in your blood and therefore affect the accuracy of your test results. Inaccurate results give your provider an inaccurate view of your health and may lead to a wrong diagnosis.

What types of blood tests require fasting?

Some examples of common blood tests that usually require fasting are:

  • Blood glucose, also called blood sugar, tests. These tests measure the level of blood glucose in a sample of your blood. They are used to help diagnose and monitor diabetes, prediabetes, and gestational diabetes (diabetes that starts during pregnancy). Not all blood glucose tests require fasting, so ask your provider if you need to fast for your test.
  • Cholesterol levels tests (lipid panel). These tests measure the level of fats, called lipids, in your bloodstream, including cholesterol and triglycerides. The test results help show your risk of getting heart disease and can be used to monitor your health after a cardiac event like a heart attack or stroke.
  • Basic metabolic panel (BMP). This is a group of tests that measures several natural chemicals in your blood. The results give your provider information about how well your organs are working, including your heart, kidneys, and liver.

In some cases, you may also be asked to fast for:

  • Liver function tests. This group of tests measures several proteins and other substances related to your liver function. Test results can give your provider information on your liver's health.
  • Renal function panel. This is a group of tests that measure several waste products and substances related to your kidney function. The results are often used to monitor your kidney function if your provider suspects kidney disease or if you have diabetes.

Whether or not your provider asks you to fast for these tests depends on several factors. For example, some liver function tests only require fasting if they're ordered as part of a comprehensive metabolic panel.

Other types of tests not listed here may require fasting. Therefore, you should always ask your provider if you aren't sure whether you should fast before your test.

Can I drink anything besides water during a fast?

No. Juice, coffee, soda, and other beverages can get in your bloodstream and affect your results. But you can drink plain water. Avoid lemon-flavored or flavored carbonated water, which may contain sugars or artificial sweeteners. Staying well hydrated can actually make the testing process a little easier. This is because water helps to keep more fluid in your veins, which can make it easier to get a sample of your blood.

Can I continue taking medicine during a fast?

Ask your provider whether it's okay to take your usual prescription medicines and/or over-the-counter medicines. For some tests, they may ask you to stop taking your medicines. But don't stop taking your medicines unless your provider tells you to stop.

Let your provider know about any vitamins and supplements that you take, too. That way you'll be sure to avoid taking anything that could affect your test results.

What if I make a mistake and have something to eat or drink besides water during my fast?

Tell your provider if you eat or drink before your test. You may need to reschedule the test for another time when you can fast to make sure you get accurate results.

When can I eat and drink normally again?

You'll be able to eat as soon as your test is over. It is common to feel light-headed after a blood test that requires fasting. So, you may want to bring a snack that you can eat right away.

Is there anything else I need to know about fasting before a blood test?

You should not fast if your provider doesn't tell you to do so. Fasting for some blood tests may also affect your results and give your provider an inaccurate understanding of your health. If you aren't sure if you should fast for a coming blood test, ask your provider as soon as possible. Taking the right steps before a blood test helps make sure your results will be accurate.

Frequently Asked Questions

Do not panic, but do not proceed with the test without informing the lab staff. Immediately notify the phlebotomist or lab technician that you broke your fast. They will assess the type of food or drink consumed and how long ago it happened. Depending on the specific test, they may reschedule the appointment or note the deviation on the lab requisition so the interpreting physician can account for potential variability in the results.
Generally, you should take your daily medications with a small sip of plain water unless your provider specifically instructs otherwise. However, certain medications like diabetes drugs, blood thinners, or supplements containing biotin can interfere with specific lab results. Always verify with your healthcare provider or pharmacist which medications are safe to take during the fasting window and which should be held until after the blood draw.
Plain water is always permitted and encouraged to keep you hydrated during the fasting period. Black coffee is a gray area; while it contains negligible calories, the caffeine and compounds in coffee can stimulate metabolic processes and potentially alter glucose or lipid levels. To ensure the highest accuracy, it is safest to stick strictly to plain water unless your provider explicitly states that black coffee is acceptable for your specific test.
Choose a light, balanced dinner that is low in fat and simple in composition, finishing the meal at least two to three hours before you go to sleep. Avoid heavy, greasy, or high-sugar meals, as these can cause prolonged elevation of triglycerides and glucose levels that may persist into the morning. Sticking to a standard, healthy meal ensures that your baseline metabolic state is as normal as possible when you arrive for the test.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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