What not to do before a B12 blood test? A guide to proper preparation

October 17, 2025 •

4 min read

According to the National Institutes of Health, up to 6% of the U.S. population may have a vitamin B12 deficiency, making accurate testing crucial. To ensure your blood test for B12 levels is as precise as possible, it's essential to understand what not to do before a B12 blood test, including avoiding certain foods, supplements, and activities that can skew your results.

Quick Summary

Preparation is critical for an accurate B12 blood test. This includes adhering to fasting instructions, temporarily stopping B12 and biotin supplements, and discussing other medications with your doctor to avoid falsely high or low readings.

Key Points

Fast for 6-12 hours: Avoid all food and drinks except plain water in the hours leading up to your test to prevent nutrient absorption from skewing results.
Stop B12 and Biotin supplements: Do not take supplemental B12 or biotin for a period before your test, as they can cause falsely high or low readings, respectively.
Discuss medications with your doctor: Inform your healthcare provider of all prescription and over-the-counter drugs, as some can interfere with B12 levels.
Avoid alcohol and smoking: Refrain from consuming alcohol and tobacco, as both can influence your vitamin B12 levels.
Skip strenuous exercise: Avoid heavy physical activity for at least 24 hours before the blood draw to ensure a rested, baseline reading.
Drink plain water: Staying hydrated with water can make the blood draw process smoother by keeping veins fuller.

The Importance of Correct B12 Blood Test Preparation

An accurate vitamin B12 blood test is vital for correctly diagnosing deficiency, which can be the cause of symptoms such as fatigue, weakness, numbness, and nerve damage. False readings, either too high or too low, can lead to misdiagnosis or unnecessary treatment. Many factors, from your diet and supplements to certain medications and lifestyle habits, can influence the concentration of B12 in your bloodstream, making proper preparation a critical step.

What to Avoid Before Your B12 Test

To ensure the reliability of your test results, several factors should be avoided in the hours or even days leading up to your blood draw. Always consult your healthcare provider for specific instructions, as they may vary depending on your medical history and specific situation.

1. Vitamin B12 and Biotin Supplements

This is one of the most critical pre-test restrictions. Consuming high doses of supplemental B12, including injections, oral supplements, sublinguals, and fortified foods, will significantly and temporarily elevate your B12 levels in the blood, leading to a falsely high reading.

How long to avoid? For B12 injections, it's recommended to wait at least a week before testing. For high-dose oral supplements and fortified foods, some sources suggest waiting at least four months to get an accurate baseline, as the body can retain unusable B12 for an extended period. For most standard tests, a shorter period might be sufficient, but always follow your doctor's specific advice.
The Biotin factor: Biotin (vitamin B7), often included in multivitamin or "hair, skin, and nail" supplements, can interfere with immunoassay technology used in many lab tests, including B12. This can result in a falsely low B12 reading. It's often advised to stop taking biotin for at least 24-72 hours before a blood test.

2. Fasting: Food, Drink (Except Water), and Caffeine

Many B12 tests require a period of fasting to ensure accuracy. The nutrients absorbed from food and drink can alter various substances in your blood, including vitamin levels. While some lab tests may not require fasting, it is a common requirement for B12 testing.

Fasting duration: You will typically be asked to fast for 6 to 12 hours before the blood draw, consuming only plain, unflavored water.
The water rule: Staying hydrated with plain water is encouraged, as it can make it easier for the phlebotomist to draw blood.
Coffee, tea, and soda: These should be avoided during the fasting period. Not only can additives like sugar and milk affect results, but caffeine itself can act as a diuretic and influence metabolism, potentially altering test readings.

3. Certain Medications

Several prescription and over-the-counter medications can affect your body's ability to absorb vitamin B12 or directly interfere with test results. It is crucial to discuss all medications with your healthcare provider well in advance of your test, but do not stop any prescribed medication without their explicit instruction.

Common culprits: Medications that can interfere include proton pump inhibitors (PPIs) like omeprazole, H2 blockers, metformin (a diabetes drug), colchicine (for gout), certain antibiotics (e.g., neomycin), and some anticonvulsants like phenytoin.
High-dose Vitamin C: Large amounts of Vitamin C taken at the same time as supplemental B12 can affect B12 levels, though this is primarily an issue with concurrent supplementation rather than a pre-test rule for naturally occurring B12.

4. Alcohol and Tobacco

Both alcohol and tobacco use can have an impact on B12 levels.

Alcohol: Chronic heavy alcohol consumption is known to lead to malabsorption and nutritional deficiencies, including B12. Excessive alcohol use in the days leading up to a test could potentially impact results.
Tobacco: Cigarette smoke contains cyanide, and the body uses B12 stores to detoxify it. This can lead to lower serum B12 levels in smokers. While the effect on a single test is less clear, it's another variable to consider. Some labs recommend no smoking for a period before testing.

5. Strenuous Exercise

Intense physical activity can temporarily alter various metabolic markers in the blood. For the most accurate and rested baseline reading, many experts recommend avoiding strenuous exercise for at least 24 hours before your blood draw.

Proper vs. Improper B12 Test Preparation


Action	Correct Preparation	Incorrect Preparation	Result on Test	Why it Matters
B12 Supplements	Discontinue high-dose supplements for several months, or as directed by your doctor.	Take a supplement the day or week before the test.	Falsely high B12 levels, hiding a deficiency.	Prevents a correct diagnosis and necessary treatment.
Biotin Supplements	Stop taking supplements for at least 24–72 hours, as directed.	Continue taking biotin as usual.	Falsely low B12 levels.	Can lead to misdiagnosis of B12 deficiency.
Fasting	Fast for 6-12 hours (water only).	Eat or drink anything other than plain water.	Inaccurate readings due to nutrient absorption.	Requires a retest and delays diagnosis.
Medication	Inform your doctor of all medications; adjust dose only if instructed.	Forget to mention a medication that affects B12 absorption.	Skewed results, either high or low, depending on the drug.	May lead to an incorrect diagnosis or treatment plan.
Alcohol	Avoid alcohol, especially heavy drinking, in the day(s) prior.	Drink alcohol heavily before the test.	Potentially suppressed B12 levels.	Can mask a deficiency or indicate one when other factors are at play.

Conclusion

Taking the proper steps to prepare for a B12 blood test is crucial for obtaining an accurate result. Avoiding B12 and biotin supplements, fasting correctly, disclosing all medications to your doctor, and abstaining from alcohol and strenuous exercise are all important considerations. By following these guidelines, you can help your healthcare provider get the most precise measurement of your B12 levels, leading to a more reliable diagnosis and appropriate treatment plan if needed. Always communicate any preparation slip-ups with your provider, as they may need to interpret the results differently or reschedule the test entirely.

Frequently Asked Questions

Fasting is often required because consuming food and drinks can introduce vitamins and nutrients into your bloodstream, temporarily elevating your B12 levels. This can lead to an inaccurate or falsely high test result.

No, you should not drink coffee or tea before a B12 blood test. Even black coffee contains compounds that can affect test results, and caffeine acts as a diuretic, which can cause dehydration and make blood drawing more difficult.

The recommended time frame can vary. For high-dose oral supplements, some experts suggest stopping for four months to allow your baseline to normalize, while for B12 injections, you should wait at least one week. Always follow your doctor's specific instructions.

Yes, high-dose biotin (vitamin B7) supplements can cause interference with laboratory assays, potentially leading to a falsely low reading for B12. You should inform your doctor and follow their instructions on when to stop taking it before the test.

If you forget and accidentally eat or drink something, you must inform the lab technician and your healthcare provider. They will need to interpret the results with that information in mind or may ask you to reschedule the test for another day.

Yes, several medications, such as certain gastric acid inhibitors, diabetes drugs like metformin, and some antibiotics, can affect B12 absorption. Never stop taking prescribed medication without your doctor's approval.

No, it is best to avoid strenuous exercise for at least 24 hours before your blood test. Physical activity can affect metabolic markers in the blood and may influence the test's accuracy.