If you’ve ever gone in for bloodwork and wondered whether your vitamins or supplements could throw off the results, it’s a smart thing to consider. Certain nutrients can cause short-term spikes in your blood, while others may interfere with how the lab actually measures your levels.

Knowing which ones matter, and when to stop them, can help ensure your test results reflect your true nutritional status, not just what you took yesterday morning.

Below, we’ll walk you through which supplements are most likely to affect results, which are less concerning, and some general guidelines you can follow.

Supplements That Can Directly Alter Blood Levels

These nutrients can cause temporary elevations if taken before your test. Pausing them for a day or two usually provides more accurate results (unless your doctor instructs otherwise).

• Vitamin B12: Serum B12 rises quickly after oral or injectable doses. Best to lay off for 24-48 hours.
• Folate (Vitamin B9): Similar to B12, recent intake can falsely elevate serum levels. Lay off for 24-48 hours.
• Iron: Oral iron can cause spikes in serum iron and transferrin saturation for several hours. Stop 24 hours before testing. However, ferritin, a storage marker, is less affected by recent iron supplementation.
• Vitamin D: Since 25(OH)D reflects long-term stores, it is unlikely to be affected by recent supplementation. However, some providers recommend skipping it the day of testing.

Supplements That Can Interfere with Testing Methods

Certain supplements don’t just alter your blood levels, they can disrupt other lab measurements.

• Biotin (Vitamin B7): Even modest doses ≥5 mg/day (or 5000 mcg) – often found in “hair, skin, and nail” or “blood sugar” formulas – can interfere with tests like thyroid, vitamin D, hormones, and certain cardiac markers. We recommend stopping 48–72 hours before drawing labs. If you’re on higher-dose biotin, you may need to pause for up to a week. Note that biotin itself doesn’t change your hormone or vitamin levels, it interferes with the test methods used in many immunoassays.

Supplements Less Likely to Skew Results

The following supplements are less likely to cause dramatic short-term fluctuations, but to play it safe, you might still skip the morning dose the day of your blood draw.

• Magnesium, calcium, zinc, selenium: Not typically altered by a single day’s intake
• Fish oil/omega-3s: Won’t affect most labs results
• Multivitamins: Skipping them 24 hours before testing can prevent confusion about how the individual nutrients in the multi may have affected your test results (like B12, folate, iron…)

Quick Reference: Supplements & Blood Testing

Other Helpful Guidelines

• If you’re tracking results over time, always prepare the same way (either always pause or always continue supplementation).
• If you’re unsure, ask your provider how they’d like you to prepare for your blood test.
• It is also important to understand which tests require fasting, so we included that information below.

Common Tests That Require Fasting

Some lab tests give the most accurate results when done on an empty stomach. “Fasting” typically means no food or drink (other than water) for 8–12 hours before your blood draw.

• Fasting glucose & HbA1c – fasting and morning testing is the best way to measure blood sugar control
• Lipid panel (cholesterol, LDL, HDL, triglycerides) – fasting is required, especially for triglycerides which are very sensitive to food intake
• Thyroid (TSH and related hormones) – fasting and morning testing provide the most accurate results since TSH fluctuates throughout the day; If you are taking desiccated thyroid (Armor, NP Thyroid) or T3 medication (such as Cytomel), wait until after your blood draw to take your medication
• Comprehensive metabolic panel (CMP) – some lab markers (like glucose) are affected by recently eating
• Iron studies – fasting and morning testing can give more consistent results since iron levels fluctuate throughout the day
• Hormone tests – your provider will specify if fasting is needed (like insulin or cortisol)