Coffee should generally be avoided during fasting for a blood test. Most guidelines advise sticking to plain water while you’re fasting, because coffee (even black) contains substances that can alter certain test results and caffeine can act as a diuretic, potentially affecting hydration and venous access for the draw. If a test is specifically assessing glucose, lipids, or other metabolic markers, any coffee with additives (milk, sugar, cream) is more likely to interfere than black coffee, and some doctors may prefer you avoid all coffee during the fasting period to ensure accuracy.

What to do now

• Check your specific test instructions: your clinician or the lab will provide guidance on what can be consumed during the fasting period for that test.

• If you’ve already consumed coffee, inform the healthcare team before the draw so they can interpret results appropriately or advise if re-testing is needed.

• For most standard fasting tests, drink only water during the fasting window (typically 8–12 hours) and resume normal beverages afterwards.

If you’d like, share the exact test name or the fasting window you were told, and a concise plan tailored to that test can be outlined.