Before your blood test
How long should you stop vitamins and supplements before a blood test?
Short answer
Most supplements do not need to be stopped, and stopping some makes the result less useful. The short list that does need a pause: high-dose biotin (hair, skin and nails products and many B-complexes) interferes with the chemistry of thyroid, troponin, hormone and vitamin D tests and should be stopped for 72 hours; an iron pill taken the morning of the draw raises serum iron for hours, so skip the dose for 24 hours before iron studies (ferritin is unaffected, and keep taking it otherwise); large doses of vitamin C can mask blood in a guaiac stool test (the FIT test is unaffected); and creatine raises creatinine without any kidney problem. Keep taking vitamin D — the test measures your level on your usual dose, which is the point. B12 supplements and injections raise serum B12 for weeks, so a deficiency question is best answered before starting them. Fish oil lowers triglycerides, which is what it is for. A typical multivitamin contains far too little biotin to interfere and can be continued.
Medically reviewed by Youmna DiStefano, MD · Last reviewed October 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| TSH, free T4/T3, troponin, hCG, vitamin D and other immunoassay hormones | interferes | High-dose biotin (hair/skin/nails products, many B-complexes): falsely high or low until it clears, 2–3 days | Stop biotin ≥72 hours before the draw [source] |
| Serum iron, transferrin saturation | raises | An iron supplement taken the same morning raises iron for hours | Skip the dose for 24 hours before iron studies; ferritin is unaffected [source] |
| Guaiac stool blood test (gFOBT) | falsely lowers | Vitamin C above a few hundred milligrams a day in the days before can mask blood; the FIT test is not affected | Stop high-dose vitamin C for 3 days before a guaiac test; no change for FIT [source] |
| Vitamin D (25-OH) | raises | The intended effect of supplementing | Keep taking it — the test measures your level on your usual dose [source] |
| Vitamin B12 (serum) | raises | Supplements and injections raise serum B12 for weeks; the level reflects the supplement, not your stores | Test before starting; if already supplementing, ask about methylmalonic acid [source] |
| Creatinine (and calculated eGFR) | raises | Creatine supplements raise creatinine while you take them, without kidney injury | Tell whoever interprets the result; see the creatine page [source] |
| Triglycerides | lowers | Fish oil (EPA/DHA) at supplement doses lowers triglycerides — the intended effect | Keep taking it [source] |
| Multivitamin (typical doses) | interferes | Not meaningfully — the biotin in a standard multivitamin is far below the interfering range | Keep taking it unless told otherwise [source] |
The short list that needs a pause — and for how long
Biotin is the one that matters most and the one people least expect. Many laboratory tests use biotin in their chemistry, and a high supplemental dose — the 5,000 to 10,000 microgram 'hair, skin and nails' products, and many B-complexes — can make a thyroid test read falsely low or high, a troponin read falsely low, and hormone and vitamin D results drift in either direction. The FDA's advice is to stop biotin before blood work, and the usual interval is 72 hours. Iron is simpler: a tablet taken that morning is being absorbed during the draw and raises serum iron and transferrin saturation for hours, so skip the dose for the 24 hours before iron studies — ferritin, the stores measure, is not affected, and you should keep taking the iron otherwise. Vitamin C in large doses can hide blood in a guaiac-based stool test, which is why those kits ask you to stop it for three days; the immunochemical FIT test is not affected. Creatine raises creatinine through muscle chemistry, not kidney damage, so a calculated eGFR can look worse than your kidneys are.
What to keep taking — stopping can make the result less useful
If the question a test is answering is 'is my dose right?', then stopping the supplement answers a different question. A vitamin D level is meant to be measured on your usual dose; stopping for a week before the draw tells you about a routine you do not follow. The same logic applies to fish oil and triglycerides, to vitamin B12 once you are supplementing, and to magnesium and zinc. A standard multivitamin can be continued: its biotin content is a small fraction of the doses linked to interference, and its other ingredients do not disturb routine chemistry. Prescription medicines are a separate matter — never stop one for a blood test without the prescriber's say-so; the medications page covers the common ones.
If you want a true deficiency baseline
Serum vitamin B12 rises within days of starting a supplement or injection and stays high for weeks, so a result taken while supplementing reflects the supplement rather than your body's stores; if you want to know whether you were deficient, test first and start after. Where that ship has sailed, methylmalonic acid is a marker that still reflects tissue B12 status. Vitamin D and iron behave differently: a level measured on treatment is informative because it shows whether the dose is working. Folate rises quickly with supplements too, so the same 'test first' rule applies.
Tests worth ordering before — or on — a supplement routine
The useful order is baseline first, supplement second, re-test after a few months on the dose. Vitamin D, B12, ferritin and iron studies are the ones a supplement routine most often starts from; a basic metabolic panel covers the creatinine a creatine user wants interpreted correctly.
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Basic Vitamin Deficiency PanelSaves $10.98 vs ordering the tests separately | Panel · 3 tests | $89.99 | No | 1–3 business days |
| Vitamin D, 25-Hydroxy | Test | $41.99 | No | 1–3 business days |
| Vitamin B12 | Test | $25.99 | No | 2–4 business days |
| Ferritin | Test | $24.99 | No | 1–3 business days |
| Iron Panel (Iron, TIBC & Ferritin) | Test · 4 biomarkers | $32.99 | Yes | 2–5 business days |
| TSH (Thyroid Stimulating Hormone) | Test | $26.99 | No | 1–3 business days |
| Comprehensive Metabolic Panel | Test · 14 biomarkers | $22.99 | Yes | 1–3 business days |
Common questions
Should I stop my multivitamin before blood work?
No. The biotin in a typical multivitamin is far below the doses that interfere with lab tests, and the other ingredients do not disturb routine chemistry. Stop only high-dose biotin or B-complex products, and skip an iron tablet the morning of iron studies.
How long before a blood test should I stop biotin?
72 hours. High-dose biotin can make thyroid, troponin, hormone and vitamin D tests read falsely high or low until it clears. Tell the lab if you could not stop it.
Can I take vitamin C before a blood test?
For blood tests, yes. The exception is a guaiac-based stool blood test, where large doses of vitamin C can mask blood — stop high-dose vitamin C for three days before that kit. The FIT stool test is not affected.
Will fish oil affect my cholesterol test?
Fish oil lowers triglycerides, which is the intended effect, and has little effect on LDL. Keep taking it — the lipid panel then shows the result of the routine you actually follow.
Should I stop vitamin D before a vitamin D test?
No. The test measures your level on your usual dose, which is exactly what you want to know. Stopping for a week beforehand tells you about a routine you do not follow.
Sources
- FDA Safety Communication — biotin interference with lab tests (2019 update)
- NIH Office of Dietary Supplements — Iron fact sheet
- MedlinePlus — Fecal occult blood test (vitamin C and diet before a guaiac test; FIT is unaffected)
- NIH Office of Dietary Supplements — Vitamin D fact sheet
- NIH Office of Dietary Supplements — Vitamin B12 fact sheet
- NIH ODS — Dietary supplements for exercise and athletic performance (creatine)
- NIH Office of Dietary Supplements — Omega-3 fatty acids fact sheet (triglyceride lowering)
- NIH Office of Dietary Supplements — Dietary supplements: what you need to know
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.