Based on NIH | Should I take my evening dose of metformin if I am fasting overnight before a blood test?

You can generally take your usual evening dose of metformin even if you are fasting overnight for routine blood tests like fasting glucose and HbA1c. Metformin does not cause low blood sugar on its own, and standard guidance emphasizes monitoring fasting glucose and HbA1c while continuing your diabetes regimen, rather than stopping metformin before these labs. ^[1] Continuing metformin supports accurate assessment of how well your treatment is controlling fasting blood sugar and long‑term glucose (HbA1c). ^[1]

Why metformin is usually continued before fasting labs

• Low hypoglycemia risk: Metformin lowers glucose mainly by reducing the liver’s glucose production and improving insulin sensitivity, and it has minimal risk of causing low blood sugar when used alone. ^[2] This mechanism means taking your evening dose does not typically interfere with a safe overnight fast. ^[2]
• Accurate treatment monitoring: Fasting blood glucose and HbA1c are standard tests used to monitor response to diabetes therapies, including metformin; these are intended to be measured while you are on your regular regimen. ^[1] Keeping doses consistent helps your clinician interpret results and adjust therapy appropriately. ^[1]

Important exceptions

There are specific situations where metformin should be temporarily withheld, but they are not routine fasting labs:

• Contrast imaging procedures: If you are scheduled for a study using iodinated contrast (for example, certain CT scans or angiography), metformin should be stopped at or before the procedure and held for 48 hours afterward, then resumed only after your kidney function is confirmed normal. ^[3] This precaution is due to rare risk of lactic acidosis when kidney function is acutely impaired by contrast. ^[3]
• Acute kidney issues or severe illness: If you have an acute condition that could impair kidney function or cause tissue hypoxia (for example, severe infection, heart failure exacerbation), clinicians may advise temporary metformin interruption. ^[4] This is unrelated to routine fasting blood draws. ^[4]

What tests are typically done while continuing metformin

• Fasting plasma glucose: Used to assess day‑to‑day control while on therapy. ^[1] Metformin lowers fasting glucose by decreasing liver glucose output, so measuring this while taking metformin shows your treatment effect. ^[2]
• HbA1c (glycosylated hemoglobin): Reflects average glucose over ~3 months, and is routinely monitored during metformin therapy. ^[1] Metformin monotherapy typically reduces HbA1c and is tracked without stopping medication beforehand. ^[5]

Practical tips for your test day

• Take your evening dose as prescribed unless your clinician has told you otherwise for a specific reason such as a contrast imaging study. ^[3] Keeping dosing consistent provides the most reliable test results. ^[1]
• Follow the fasting instructions for food and drink (usually nothing but water for 8–12 hours before the draw), and bring your morning dose to take right after the blood is collected if your provider prefers morning dosing timing. ^[1]
• Confirm with your lab or clinic if your appointment involves contrast imaging or special procedures; if so, you may need to withhold metformin as noted above. ^[3]

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Quick comparison: routine fasting labs vs. contrast procedures

Scenario	Should you take metformin the evening before?	Rationale
Routine fasting blood tests (fasting glucose, HbA1c)	Yes, take as usual	These tests monitor control while on therapy; metformin is not typically withheld. [1]
Imaging with iodinated contrast (e.g., CT with contrast, angiography)	No; hold at/before procedure, then for 48 hours after	Contrast can acutely affect kidneys; metformin is paused to avoid rare lactic acidosis risk and resumed after confirming normal renal function. [3]

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Key takeaways

• Most people should take their evening metformin before an overnight fast for blood tests. This supports accurate monitoring and has minimal risk of low blood sugar. ^{[1] [2]}
• Only withhold metformin for certain procedures, especially iodinated contrast studies, and restart after kidney function is checked. ^[3]

If you’re unsure whether your appointment includes contrast imaging or a special protocol, a quick call to your clinic can clarify the plan.