Before your blood test
Does metformin affect blood test results — and should you take it before a blood test?
Short answer
Take your metformin as usual unless the prescriber or the lab has told you otherwise — holding a dose for a routine draw is not needed, and a fasting draw is simply taken before the morning dose. Metformin changes results in a few predictable ways: over months to years it lowers vitamin B12 (and can raise methylmalonic acid before B12 falls), which is why B12 is checked periodically on long-term metformin; it lowers fasting glucose and A1c, which is the point; and it is dosed by kidney function, so eGFR is checked at least yearly and before iodinated contrast scans. It does not interfere with the assays themselves.
Medically reviewed by Youmna DiStefano, MD · Last reviewed October 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| Vitamin B12 | lowers | Develops over months to years of use; dose-related | Check B12 (and MMA if borderline) periodically on long-term metformin, sooner with tingling, anemia or a high MCV [source] |
| Methylmalonic acid (MMA) | can raise | As B12 stores fall, often before serum B12 is clearly low | The more sensitive follow-up when B12 is 200–300 pg/mL [source] |
| Glucose (fasting) and A1c | lowers | Glucose within days; A1c over ~3 months | Expected; take the dose as usual — a fasting draw happens before the morning dose [source] |
| eGFR / creatinine | interferes | Not changed by the drug — but the drug is dosed by it | eGFR at least yearly; metformin is held around iodinated contrast if eGFR is reduced [source] |
| Lactate | can raise | Rare; with kidney failure, dehydration, heavy alcohol or acute illness | Not a routine test; relevant if acutely unwell on metformin [source] |
Should you take metformin before a fasting blood test?
Yes, in the normal way: a fasting draw is scheduled first thing, you take the morning dose with breakfast afterwards. Metformin does not raise or lower glucose acutely the way insulin or a sulfonylurea can, so there is no low-blood-sugar risk in fasting on it and no reason to skip it. If your prescriber wants to see glucose off the drug, that is a separate conversation — do not stop it on your own for a test.
The one situation where metformin is held is a scan with iodinated contrast when kidney function is reduced, and your care team manages that timing.
Which results does metformin change?
The important one is vitamin B12. Metformin reduces B12 absorption, and after years of use a meaningful share of people run low — often without obvious anemia, sometimes with numbness or tingling that gets blamed on diabetes itself. Guidelines recommend periodic B12 checks on long-term metformin; a B12 in the gray zone (roughly 200–300 pg/mL) is clarified with methylmalonic acid, which rises as B12 stores fall. Otherwise the changes are the intended ones — lower fasting glucose and A1c — and the kidney check is about dosing, not damage: metformin itself does not injure the kidney, but it is cleared by it, so eGFR is monitored at least yearly.
The tests this affects — order without a doctor's visit
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 |
|---|---|---|---|---|
| Vitamin B12 | Test | $25.99 | No | 2–4 business days |
| Methylmalonic Acid (MMA) | Test | $49.99 | No | 5–12 business days |
| Hemoglobin A1c | Test | $21.99 | No | 2–4 business days |
| Comprehensive Metabolic Panel | Test · 14 biomarkers | $22.99 | Yes | 1–3 business days |
Common questions
Can I take metformin before a fasting blood test?
Yes. Fast overnight, have the draw, then take your morning dose with breakfast. Metformin does not cause low blood sugar on its own, so there is no need to skip it.
Does metformin cause B12 deficiency?
It lowers B12 absorption, and over years a meaningful share of people become deficient. Periodic B12 checks are recommended on long-term metformin; methylmalonic acid clarifies a borderline result.
Why does my doctor check kidney function on metformin?
Metformin is cleared by the kidneys and dosed by eGFR, so eGFR is checked at least yearly. The drug does not damage kidneys; the check is about whether the dose is still right.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.