Plain water is the only drink universally approved during a fast before blood work. No juice, coffee, tea, soda, or flavored water. The goal of fasting is to clear your bloodstream of anything that could skew your results, and nearly every beverage besides plain water introduces something that can do exactly that.
Why Only Plain Water
When your provider orders a fasting blood test, they need a clean baseline reading of markers like blood sugar, triglycerides, and cholesterol. Anything you consume other than water can change those numbers. Even small amounts of sugar, fat, caffeine, or plant compounds from beverages get absorbed into your bloodstream and can shift the results enough to make a normal value look abnormal, or hide a genuinely high one.
MedlinePlus, the National Institutes of Health’s patient resource, puts it simply: you should not eat or drink anything except plain water for the required fasting window. That typically means 10 to 12 hours before a lipid panel and 8 to 12 hours before a fasting glucose test. Most people schedule morning blood draws and do the bulk of their fast overnight.
What About Black Coffee?
This is the most common question, and the answer is more nuanced than most people expect. A small study of 10 healthy adults found that 8 ounces of black coffee produced no meaningful change in fasting blood sugar compared to water, with a difference of just 0.4 mg/dL. For a glucose-only test, that’s essentially noise.
Lipid panels are a different story. Coffee contains natural oils called diterpenes that can raise total cholesterol and LDL cholesterol. Data from a large national survey found that each additional daily cup of coffee was associated with a 1.23 mg/dL increase in total cholesterol and a 1.22 mg/dL increase in LDL. Among people who drink three or more cups a day, those numbers jumped to roughly 8 mg/dL per additional cup. If your provider ordered a lipid panel, coffee before the draw could nudge your cholesterol reading higher than your true baseline.
The safest approach is to skip the coffee entirely. If you’ve already had a cup of black coffee before a fasting glucose test, it probably won’t ruin the results. But for a comprehensive metabolic panel or lipid panel, stick with water.
Drinks That Seem Safe but Aren’t
Several beverages trip people up because they feel “close enough” to water.
- Lemon water or fruit-infused water: Even a squeeze of lemon introduces citric acid and trace sugars. Cleveland Clinic specifically advises against water with lemon or lime added before fasting blood work.
- Flavored sparkling water: Many brands contain artificial sweeteners, natural flavors, or both. MedlinePlus warns against lemon-flavored or flavored carbonated water because of hidden sugars and sweeteners.
- Diet soda and zero-calorie drinks: While artificial sweeteners on their own don’t appear to raise blood sugar directly, these beverages often contain other ingredients (acids, colorings, caffeine) that can interfere with blood chemistry.
- Tea: Plain black or green tea contains caffeine and polyphenols that interact with liver enzymes and iron metabolism. Even unsweetened, tea is not considered equivalent to water for fasting purposes.
- Bone broth or clear broths: These contain protein, fat, and electrolytes that will break your fast.
Unflavored, unsweetened plain sparkling water (just carbonated water with no additives) falls into a gray area. Some labs consider it acceptable, but the official guidance from MedlinePlus and Cleveland Clinic focuses exclusively on plain water. If you want to play it safe, flat water is the best choice.
How Much Water You Should Drink
Drinking water during your fast isn’t just allowed, it’s encouraged. Being well-hydrated makes your veins easier to find, which means a quicker, less painful draw and fewer attempts with the needle. Dehydration can also slightly concentrate certain blood markers, potentially affecting results.
You don’t need to overdo it. A glass or two of water in the hour before your appointment is enough. Drink normally through the evening before, then have some water when you wake up.
Morning Medications During a Fast
Most routine medications should still be taken on schedule with a small sip of plain water, but this depends on the specific test and the specific medication. Some drugs directly affect blood sugar, liver enzymes, or lipid levels. If you take daily prescriptions, ask your provider when the test is ordered whether to take them before or after the blood draw. Don’t skip a medication on your own just because you’re fasting.
What Happens if You Accidentally Break the Fast
If you forgot and had your morning coffee, a splash of creamer, or a piece of toast, tell the person drawing your blood. In many cases, the lab can still run the tests and note that you weren’t fully fasting, or your provider may ask you to reschedule. A non-fasting result isn’t useless, but it can’t be compared accurately to fasting reference ranges. Being honest about it saves everyone time and avoids a false alarm over an elevated reading that was really just your breakfast.