Medically reviewed by Biljana Srbinovska, MPharm. Last reviewed July 2026.
In almost every case you should keep taking your regular medications before a fasting blood test, with a small sip of water. Stopping them is usually the riskier choice. There are a few genuine exceptions worth knowing: diabetes medication, where fasting creates a real hypoglycaemia risk, levothyroxine, which should be taken after the draw rather than before, and biotin supplements, which can distort a wide range of test results including heart attack tests. Here is what to do.
What Fasting Actually Means
No food or drink except plain water, usually for 8 to 12 hours. Water is allowed and encouraged, since being hydrated makes the blood draw easier.
Tests that usually require fasting include a lipid panel, fasting glucose, glucose tolerance testing, fasting insulin, and iron studies. Tests that do not require fasting include HbA1c, full blood count, thyroid function, and liver function tests on their own.
Worth knowing: guidance on lipid testing has moved. Major European and US guidelines now accept a non-fasting lipid panel for routine risk assessment, with a repeat fasting sample only if triglycerides come back very high. If fasting is difficult for you, it is reasonable to ask whether it is necessary for your particular test.
The General Rule on Medication
Take your usual medicines as normal with a sip of water unless your doctor has told you otherwise. Skipping blood pressure medication, anticoagulants, or antiepileptics to prepare for a blood test carries far more risk than any effect on the result.
Exceptions fall into four groups: drugs whose blood level is being measured, diabetes medication, levothyroxine, and biotin.
Diabetes Medication: The Real Safety Issue
This is the exception that matters most, and it deserves a conversation with your prescriber rather than a decision made from an article.
Insulin and sulfonylureas such as gliclazide and glipizide lower blood sugar regardless of whether you have eaten. Taking a normal dose and then fasting for twelve hours creates a genuine risk of hypoglycaemia. Standard practice is to reduce or hold the dose while fasting and to monitor your blood sugar.
Metformin does not cause hypoglycaemia on its own, so it is generally less of a concern, though it is sometimes held for stomach upset.
Practical steps if you take diabetes medication: book the earliest appointment of the day so the fast is shortest, ask your prescriber specifically what to do with your dose, check your blood sugar before leaving home, and take your medication and something to eat with you for straight after the draw.
If you feel shaky, sweaty, confused, or unwell while fasting, treat it as hypoglycaemia and eat. A rescheduled blood test is a minor inconvenience. A hypoglycaemic episode is not.
Levothyroxine: Take It After
If you take thyroid hormone replacement, take it after your blood is drawn rather than before.
The reason is specific. Free T4 rises for a few hours after a dose and can read meaningfully higher than your true level. TSH is unaffected, because it changes slowly. So a same-morning dose risks making your levels look higher than they are, and a dose reduction you did not need.
Biotin: The Supplement That Distorts Results
This is the most important thing on this page, because most people do not think of a hair and nail supplement as a medication.
Biotin interferes with a large family of laboratory tests. The FDA issued a safety communication in November 2017 and updated it in 2019, prompted in part by a reported death where biotin caused a falsely low troponin result, the test used to diagnose a heart attack.
Affected tests include troponin, thyroid function, parathyroid hormone, pregnancy tests, PSA, ferritin, folate, cortisol, and vitamin B12. Results can be falsely high or falsely low depending on the test.
The doses matter. The daily requirement is a fraction of a milligram, while hair, skin, and nail supplements commonly contain 5 to 20 milligrams, and some products contain far more.
One correction worth noting. Many articles state you should stop biotin 72 hours before testing and attribute this to the FDA. The FDA explicitly says the opposite, that there is not enough information to know whether stopping for any particular period prevents incorrect results. The 72-hour figure comes from a laboratory medicine association, which suggests at least 8 hours for doses of 5 to 10 milligrams a day and at least 72 hours for 100 milligrams or more.
The practical advice: tell the laboratory and your doctor that you take biotin. That is more useful than any stopping interval, because they can interpret or repeat the test accordingly.
Other Things That Affect Results
Alcohol. Avoid for 24 hours, since it affects liver tests, triglycerides, and glucose.
Coffee. Even black coffee breaks a strict fast according to some laboratories. Water only is the safe answer.
Chewing gum, including sugar-free, stimulates digestion and is best avoided.
Vigorous exercise in the 24 to 48 hours beforehand can raise muscle and liver enzymes enough to mimic muscle or liver disease. If you have done an unusually hard workout, mention it.
Smoking before the draw is best avoided.
If You Slip Up
Tell the phlebotomist. Do not conceal it.
The usual consequence is that you come back another day, which is a minor inconvenience. The consequence of staying quiet is a result that gets acted on clinically when it should not be, which is a much bigger problem.
Fasting Blood Test FAQs
Should I take my medication before a fasting blood test?
Generally yes, with a sip of water, unless your doctor says otherwise. The main exceptions are diabetes medication, levothyroxine, and any drug whose blood level is being measured.
What about my diabetes medication?
Ask your prescriber before the test. Insulin and sulfonylureas lower blood sugar whether or not you have eaten, so fasting creates a real hypoglycaemia risk. Book the earliest appointment and bring your medication and food for afterwards.
Can I take levothyroxine before the test?
Take it after the blood draw. A dose taken beforehand can raise your free T4 reading and lead to an unnecessary dose reduction.
Do I need to stop biotin supplements?
Biotin distorts many tests including troponin and thyroid tests, and the FDA has issued a safety warning. The FDA does not endorse a specific stopping interval. The most useful step is to tell the laboratory and your doctor that you take it.
Can I drink coffee?
Water only is safest. Some laboratories consider even black coffee to break a fast, and it is not worth risking a repeat visit.
Do I still need to fast for a cholesterol test?
Often not. Major guidelines now accept non-fasting lipid testing for routine risk assessment, with a fasting repeat only if triglycerides are very high. Ask whether fasting is needed for your specific test.
