Before blood work, you should avoid eating (if fasting is required), drinking anything besides plain water, exercising heavily, consuming alcohol, and taking certain supplements like biotin or high-dose vitamin C. Each of these can skew your results in specific ways, potentially leading to a retest or, worse, a misdiagnosis. The exact precautions depend on which tests you’re getting, but a few rules apply broadly.
Don’t Eat or Drink the Wrong Things
If your provider orders a fasting blood test, you need to avoid all food and beverages except plain water for 8 to 12 hours beforehand. This applies most often to blood glucose tests and lipid panels, which measure cholesterol and triglycerides. Even small amounts of sugar or fat entering your bloodstream can throw off these numbers.
The “plain water” rule is stricter than most people realize. Coffee, juice, soda, tea, and flavored water are all off limits, even if they’re sugar-free. Lemon-flavored or flavored sparkling water can contain sugars or artificial sweeteners that affect results. Black coffee is a common question, and the answer is no: it can alter blood chemistry enough to matter.
One thing worth knowing: fasting isn’t always required for cholesterol screening anymore. The American College of Cardiology and American Heart Association updated their guidelines in 2018 to allow non-fasting lipid testing for routine screening. So if your provider didn’t specifically tell you to fast, you may not need to. When in doubt, call the lab or your provider’s office the day before.
Don’t Exercise Hard the Day Before
A tough workout can change your blood chemistry for longer than you’d expect. When you do heavy strength training or intense cardio, muscle enzymes leak into your bloodstream. One key marker, creatine kinase, peaks about 24 hours after damaging exercise and can stay elevated for up to seven days. If your doctor is checking for muscle or organ damage, a hard gym session could make it look like something is wrong when you’re perfectly healthy.
Exercise also temporarily raises cortisol (your body’s stress hormone) and triggers inflammatory markers that can take anywhere from minutes to hours to return to normal. For the most accurate baseline results, avoid strenuous exercise for at least 24 hours before your blood draw. Light walking or gentle stretching is fine.
Don’t Drink Alcohol
Alcohol hits several blood markers at once. It raises triglycerides in a dose-dependent way, meaning the more you drink, the higher they go. Heavy drinking can push triglyceride levels well above normal range, which would flag you for cardiovascular risk that might not reflect your everyday health. Alcohol also elevates liver enzymes, particularly GGT, which is especially sensitive to heavy drinking. In research on binge drinking patterns, the heaviest drinkers had GGT levels over eight times more likely to be clinically high compared to non-binge drinkers.
Skip alcohol for at least 24 hours before blood work. If your tests include a liver panel, 48 to 72 hours is safer, since liver enzymes can stay elevated longer than other markers.
Don’t Take Biotin Supplements
Biotin is one of the most disruptive supplements you can take before lab work, and most people have no idea. It’s found in hair, skin, and nail supplements, B-complex vitamins, and prenatal vitamins, sometimes at doses far above what you’d get from food. The FDA has issued warnings that biotin can significantly interfere with lab tests and cause incorrect results that may go undetected.
The interference is especially dangerous for troponin tests, which diagnose heart attacks. Biotin can cause falsely low troponin readings, potentially masking a cardiac emergency. It also affects thyroid panels and hormone tests. If you take any supplement containing biotin, stop it at least 72 hours before your blood draw. Check the label on your multivitamin too, since biotin is a common ingredient.
Don’t Take High-Dose Vitamin C
Large doses of vitamin C can interfere with blood glucose readings. Vitamin C is a strong antioxidant that reacts with the electrochemical strips used in glucose meters, producing extra electrical current that gets read as higher blood sugar. At concentrations of 15 mg/dL or above in the blood, some glucose meters showed unacceptable accuracy, with deviations greater than 15% from the true value. That’s enough to trigger a false diabetes diagnosis or mask actual low blood sugar.
If you take vitamin C supplements (especially at doses of 500 mg or more), skip them for at least 24 hours before any test that involves glucose measurement.
Don’t Smoke or Use Nicotine
Nicotine triggers your sympathetic nervous system, causing a spike in heart rate and blood pressure. It also releases stress hormones that can affect glucose levels and other metabolic markers. If your blood draw involves any cardiovascular screening or metabolic panel, nicotine use right before the test introduces noise into your results. Avoid cigarettes, vapes, and nicotine patches for at least a few hours before your appointment.
Don’t Skip Water
While you need to avoid most beverages before a fasting test, plain water is not only allowed but encouraged. Dehydration is a major source of lab result variability. When you’re dehydrated, your blood becomes more concentrated, which can artificially inflate values for things like red blood cell counts, protein levels, and other markers. Researchers have noted that even small percentage changes in hydration can shift results enough to change their clinical interpretation, particularly for tests with narrow reference ranges.
Dehydration also makes veins harder to find. If you’ve ever had a phlebotomist struggle to draw your blood, being better hydrated next time can make a real difference. Drink a few glasses of water the evening before and another glass or two the morning of your test.
If You Accidentally Eat or Drink Something
If you forget and eat breakfast or drink coffee before a fasting test, don’t try to hide it. Tell the lab technician or your provider. In most cases, you’ll simply need to reschedule. Running the test anyway risks inaccurate results that could lead to unnecessary follow-up tests or missed diagnoses. It’s a minor inconvenience compared to getting unreliable numbers in your chart.
For medications, the rules vary. Some prescriptions need to be taken on schedule regardless of fasting. Others, like diabetes medications that lower blood sugar, might need to be held until after the draw. Your provider should give you specific instructions when ordering the test. If they didn’t mention it and you take daily medications, ask before skipping any doses on your own.