Before a fasting blood test, you should not eat or drink anything except plain water for the number of hours your provider specifies, typically 8 to 12. But food isn’t the only thing that can throw off your results. Exercise, alcohol, smoking, supplements, and even dehydration can alter specific markers in ways that lead to inaccurate readings or unnecessary follow-up testing.
Don’t Eat or Drink Anything Besides Water
The most common instruction is to fast, and fasting means nothing but plain water. Juice, coffee, soda, flavored water, and even lemon-flavored sparkling water can introduce sugars, artificial sweeteners, or other compounds into your bloodstream. The tests that typically require fasting are blood glucose tests, cholesterol panels (also called lipid panels), and basic metabolic panels. You may also need to fast for liver function tests or kidney function panels, especially when they’re part of a comprehensive metabolic panel.
If you accidentally eat or drink something before a fasting blood draw, don’t panic. Let the person drawing your blood know what happened. In most cases, the results can still be interpreted with that context, or your provider will simply reschedule. For a blood sugar test, for instance, a nonfasting result can still be read using different reference ranges. That said, if you have diabetes or a condition like a vitamin deficiency where precise values matter, it’s especially important to flag the mistake so your care team can decide whether to redraw.
Skip Your Morning Workout
Strenuous exercise before a blood draw can distort several important markers. Physical activity causes microscopic tearing in muscle fibers, which releases enzymes normally found inside muscle cells into the bloodstream. One of these, creatine kinase, is a well-known marker for muscle or heart damage. If it shows up elevated on your results because you went for a hard run that morning, it could trigger unnecessary concern or additional testing.
Exercise also raises levels of enzymes traditionally used to assess liver health. These enzymes exist in skeletal muscle too, so post-workout muscle damage causes them to spill into the blood, mimicking what might look like a liver problem. On top of the direct muscle effect, exercise temporarily alters blood flow to the liver itself, which can further skew those numbers. To keep your results clean, avoid vigorous activity for at least 24 hours before your blood draw.
Avoid Alcohol for at Least 24 Hours
Alcohol has a surprisingly broad impact on blood work. Even a night of moderate drinking can elevate triglycerides and alter your cholesterol profile. Alcohol also raises HDL cholesterol, which might sound like a good thing, but it masks your true baseline and makes a lipid panel harder to interpret accurately.
Liver enzyme markers are particularly sensitive to alcohol. In regular drinkers, one key liver enzyme can take four to five weeks to return to normal after stopping. Other liver enzymes need two to four weeks of abstinence to normalize. For a routine blood test, you don’t need weeks of sobriety, but avoiding alcohol for at least 24 to 72 hours gives your body time to clear the acute effects that are most likely to skew results.
Don’t Smoke Before Your Draw
Smoking a cigarette before a blood test raises your white blood cell count for at least an hour afterward. Lymphocyte and granulocyte counts, two specific types of white blood cells, both increase significantly. This means your results could suggest your immune system is fighting an infection or dealing with inflammation when it’s really just responding to the cigarette you had in the parking lot. Even secondhand smoke exposure has been shown to bump up these same markers. If you’re a smoker, hold off until after your draw.
Hold Off on Biotin Supplements
Biotin, the B vitamin found in hair, skin, and nail supplements, is one of the most common causes of misleading blood test results. At doses above 5 mg per day, biotin interferes with the laboratory technology used for many common tests, including thyroid panels and cardiac markers. The interference doesn’t change what’s actually happening in your body; it tricks the testing equipment into producing inaccurate numbers.
Many over-the-counter beauty supplements contain biotin at doses of 5 to 10 mg, well above the level that causes problems. Stop taking biotin at least 48 to 72 hours before any blood test, particularly if thyroid function is being checked. If you’re not sure whether your multivitamin or supplement contains biotin, check the label or bring the bottle with you when you ask your provider.
Tell Your Provider About Medications
You should never stop a prescribed medication without your doctor’s guidance, but you should let your provider know everything you’re taking, including over-the-counter drugs and supplements. Many common medications alter blood test values in ways that can mimic disease or obscure a real problem. High-dose vitamin C, for example, can artificially lower cholesterol and triglyceride readings while raising creatinine, a kidney function marker. Common pain relievers and acid-reducing medications can also interfere with specific tests.
The key isn’t necessarily to stop these medications. It’s to make sure your provider knows about them so results can be interpreted correctly. Ideally, share your full medication and supplement list at least a few days before your test. Your provider can tell you whether anything needs to be paused and for how long.
Think Twice About Black Coffee
Many people assume black coffee is safe during a fasting period since it has no calories. For most healthy adults, caffeine doesn’t noticeably affect blood sugar, so plain black coffee may not ruin a basic glucose test. However, if you have diabetes, caffeine can alter the way your body uses insulin, potentially pushing blood sugar higher or lower depending on individual sensitivity. Doses as low as 200 milligrams (roughly one strong cup) can cause this shift in some people with diabetes.
The safest approach is to stick with water only. Coffee is also mildly dehydrating, which introduces another variable. If your provider specifically said “nothing but water,” take that literally.
Don’t Show Up Dehydrated
Dehydration concentrates your blood. When you have less fluid in your bloodstream, the relative concentration of everything dissolved in it goes up: sodium, proteins, red blood cells, and other markers all appear elevated compared to their true baseline. This can make normal values look abnormal or push borderline results over the threshold. Dehydration also makes veins harder to find, which can turn a quick draw into a more difficult and uncomfortable experience.
Drink water normally in the hours leading up to your test. You don’t need to overhydrate, but sipping water throughout the morning, especially if you’re fasting and skipping your usual coffee or juice, keeps your blood volume where it should be and gives the lab the most accurate snapshot of your health.
Don’t Chew Gum
It seems harmless, but gum is on the “do not” list for fasting blood tests. Even sugar-free varieties contain sweeteners that could affect results. Research shows that chewing itself alters levels of certain gut hormones involved in appetite and blood sugar regulation, even when glucose and insulin levels stay stable. While the effect is subtle, it introduces an unnecessary variable. Mints fall into the same category. If you need to freshen your breath before your appointment, plain water is the only safe option.