Before a blood test, the main things to avoid are eating (if you’ve been told to fast), drinking coffee or alcohol, taking certain supplements, and doing intense exercise. Most of these can directly alter the markers your doctor is trying to measure, leading to results that look abnormal when you’re actually fine, or normal when something needs attention.
Eating During a Fasting Window
If your doctor orders a fasting blood test, you’ll typically need to go without food for 8 to 12 hours beforehand. The most common tests that require fasting are blood glucose (blood sugar), cholesterol panels, and basic metabolic panels. Eating during this window can spike your blood sugar and triglycerides, making your results unreliable.
That said, not every blood test requires fasting. A 2026 guideline from the American College of Cardiology and American Heart Association notes that LDL cholesterol levels usually differ very little between fasting and non-fasting states, so many doctors now accept non-fasting samples for routine cholesterol checks. Fasting still matters most if you have a history of high triglycerides (particularly above 400 mg/dL) or a family history of genetic cholesterol disorders. If you’re unsure whether your specific test requires fasting, ask your provider rather than assuming.
Drinking Coffee, Even Black
Plain water is fine and even encouraged before a blood draw. It keeps your veins easier to find and makes the process smoother. But coffee is a different story. Cleveland Clinic advises against drinking any coffee while fasting for blood work, including black coffee with no sugar or cream. Caffeine can affect blood sugar metabolism and stress hormone levels, both of which skew results. Tea falls into the same category. Stick to water only.
Drinking Alcohol
Alcohol raises triglyceride levels and affects liver enzyme markers, two things doctors frequently check in routine bloodwork. You should avoid alcohol for at least 24 hours before a lipid panel. Even a couple of drinks the night before can inflate your triglyceride reading enough to make it look like you have a metabolic problem you don’t actually have. If your blood test includes liver function markers, the same 24-hour rule applies, since alcohol temporarily elevates the enzymes those tests measure.
Intense Exercise
A hard workout within 24 hours of your blood draw can throw off a surprising number of markers. Research published in the Journal of Clinical Medicine found that endurance exercise significantly raised levels of creatine kinase (a muscle damage marker), liver enzymes, creatinine, uric acid, glucose, cholesterol, and triglycerides. At the same time, it lowered potassium and calcium. These shifts happen partly because exercise physically stresses tissues and partly because you lose plasma volume through sweat, concentrating everything in your blood.
A light walk or gentle stretching won’t cause problems. The concern is vigorous activity: long runs, heavy weight training, intense cycling, or competitive sports. If you have a blood test scheduled for the morning, skip your hard workout the day before or at least dial it back significantly.
Taking Biotin Supplements
Biotin (vitamin B7) is one of the most common supplement-related causes of false lab results, and most people don’t know about it. It’s found in hair, skin, and nail supplements, often at doses of 5 to 10 mg. At those levels, biotin interferes with the technology used in many immunoassays, which are the tests that measure thyroid hormones, heart damage markers, and other critical values. The result can be a falsely normal or falsely abnormal reading.
Regular multivitamins with up to 1 mg of biotin haven’t been reported to cause interference. But if you take a dedicated biotin supplement at 5 to 10 mg, the Association for Diagnostics and Laboratory Medicine recommends waiting at least 8 hours after your last dose before having blood drawn. For people on high-dose biotin therapy (100 mg or more per day, sometimes prescribed for neurological conditions), the recommended washout period is at least 72 hours. The simplest approach: stop your biotin supplement two to three days before your blood test.
Changing Your Medications Without Asking
Some medications affect the same markers your blood test is measuring. Certain drugs raise or lower blood sugar, alter cholesterol readings, or change liver enzyme levels. But the important rule here is not to stop or skip any medication on your own. Tell your provider what you’re currently taking, including over-the-counter drugs, vitamins, and supplements, and let them decide if anything needs to be adjusted or timed differently around your blood draw. Skipping a dose of a prescribed medication without guidance can be more dangerous than a slightly skewed lab result.
Chewing Sugary Gum
Sugar-free gum is mostly fine. A study of 12 fasting people found that chewing sugar-free gum for 30 minutes did not affect insulin levels, and a separate study involving 59 people with gestational diabetes showed no effect on blood sugar after chewing gum. But gum with added sugar could technically raise your glucose during a fasting test, especially if you chew several pieces. If you want to be safe, skip the gum entirely on the morning of your test.
Smoking Before Your Draw
Smoking raises blood pressure, heart rate, and cortisol levels, all of which can affect certain test results. Nicotine also temporarily changes white blood cell counts and carbon monoxide levels in your blood. If you can, avoid smoking for at least 30 minutes before your appointment. For tests that measure inflammation or stress hormones, a longer break is better.
Quick Reference: Timing Guidelines
- Food: Stop eating 8 to 12 hours before a fasting test
- Alcohol: Avoid for at least 24 hours
- Coffee and tea: Avoid the morning of your test
- Biotin supplements (5+ mg): Stop at least 8 hours before, ideally 2 to 3 days
- Intense exercise: Avoid for 24 hours
- Water: Drink normally, and even a bit extra to stay well hydrated