Fasting is not strictly required for a vitamin B12 blood test, but most labs and doctors prefer it. The standard recommendation is to avoid eating for 6 to 8 hours before your blood draw, with water still allowed during that time. In practice, this means scheduling a morning appointment and skipping breakfast is the easiest approach.
Why Labs Prefer a Fasting Sample
Eating a meal doesn’t dramatically change your B12 blood levels the way it changes blood sugar or cholesterol. Research on postprandial (after-eating) blood samples shows that B12 concentrations shift only slightly after food, not enough to push a result from normal into deficient or vice versa. The change is typically small enough that it wouldn’t alter a doctor’s clinical interpretation.
So why do labs still ask you to fast? The main reason is that eating, especially fatty foods, can make your blood sample lipemic, meaning it looks milky from fat particles. Lipemic samples can be harder for lab instruments to process accurately, which introduces the possibility of error. Fasting for at least 8 hours eliminates that problem. Labcorp, one of the largest commercial laboratories, lists a fasting specimen as “preferred” for its B12 test.
What Counts as Fasting
When your doctor or lab says to fast, they mean no food, juice, coffee, or other beverages besides plain water. You can drink water freely, and staying hydrated actually makes the blood draw easier because your veins are fuller. If you take daily medications, ask your doctor whether to take them as usual or wait until after the draw.
The simplest approach: stop eating after dinner the night before, drink water in the morning, and get your blood drawn first thing. That easily covers the 6 to 8 hour window.
Biotin Supplements Can Skew Results
One preparation step that matters more than fasting is stopping biotin supplements before your test. Biotin (vitamin B7) is found in many hair, skin, and nail supplements, sometimes at doses of 5 to 10 mg. At those levels, biotin interferes with the lab technology used to measure B12, and the interference pushes your result artificially higher. That means you could appear to have normal B12 when you’re actually deficient.
If you take a biotin supplement at doses between 5 and 10 mg, stop it at least 8 hours before your blood draw. If you take high-dose biotin therapy (100 mg per day or more, sometimes prescribed for conditions like multiple sclerosis), you need to stop at least 72 hours before testing. Standard multivitamins usually contain only tiny amounts of biotin (30 to 100 micrograms), which are unlikely to cause interference, but it’s worth checking the label.
Other Tests That May Be Ordered Alongside B12
If your doctor suspects B12 deficiency, they may also order tests for methylmalonic acid (MMA) and homocysteine. These markers rise when your body doesn’t have enough functional B12, so they can catch a deficiency earlier than the standard B12 blood test alone. MMA testing may also require fasting for several hours beforehand, so the same morning-appointment strategy works well if your doctor orders both.
Folate (vitamin B9) is another test commonly bundled with B12 because the two vitamins work together and their deficiencies can look similar. Folate testing also typically calls for a fasting sample.
Medications That Affect B12 Levels
Certain medications lower your B12 levels over time, which is worth knowing before and after testing. The most common culprits are metformin (widely prescribed for type 2 diabetes), proton pump inhibitors and other antacids (used for acid reflux), and repeated exposure to nitrous oxide. These drugs don’t interfere with the test itself the way biotin does. Instead, they genuinely reduce how much B12 your body absorbs, so your result reflects a real change.
You don’t need to stop these medications before testing. In fact, knowing your B12 level while taking them is the whole point, since long-term use is one of the main reasons doctors order the test. Just make sure your doctor knows what you’re taking so they can interpret results in context.
Who Should Get Tested
B12 testing is most useful when there’s a specific reason to suspect deficiency rather than as a routine screening for everyone. Clinical guidelines recommend testing if you follow a strict vegan or vegetarian diet, have had stomach or intestinal surgery, live with inflammatory bowel disease, take metformin or acid-suppressing medications long term, or are over 75 years old. Symptoms that often prompt testing include persistent fatigue, numbness or tingling in the hands and feet, difficulty with balance, brain fog, and mood changes.
Ordering B12 tests without a clear reason can lead to borderline results that trigger unnecessary follow-up without improving your care. If you’re curious about your levels but don’t have symptoms or risk factors, it’s worth discussing with your doctor whether the test would actually change your treatment plan.