Water is the only universally safe drink during a fasting period before blood work. In fact, drinking plenty of water is actively encouraged. Most other beverages, including juice, milk, soda, and flavored water, will break your fast and can distort your results. The typical fasting window is 8 to 12 hours before your blood draw, depending on the test.
Why Fasting Matters for Blood Tests
When you eat or drink anything with calories, your body breaks it down into sugars and fats that enter your bloodstream. Triglyceride levels, for example, can rise 19 to 30 percent above fasting levels within a few hours of a meal. If your blood is drawn during that window, your results won’t reflect your baseline levels, and your doctor could misinterpret them as a sign of a problem that isn’t there.
The two most common tests that require fasting are lipid panels (cholesterol and triglyceride checks) and blood glucose tests. Not every glucose test requires fasting, so check with your provider if you’re unsure. Other tests, like a basic metabolic panel, may also call for it.
Water: Your Best Option
Plain water is the gold standard. It has zero effect on blood sugar, triglycerides, or any other marker being tested. Beyond being “allowed,” it actually makes the blood draw easier and faster. Well-hydrated veins are plumper and closer to the surface, which means the phlebotomist can find them more easily and you spend less time in the chair. Ideally, start hydrating well the day before your appointment, not just the morning of.
What About Black Coffee or Plain Tea?
This is where it gets nuanced. One controlled study found that 8 ounces of black coffee consumed 30 minutes before a blood draw produced no meaningful change in fasting glucose or triglycerides compared to water alone. The differences were tiny, less than 1 mg/dL for glucose and under 2 mg/dL for triglycerides.
However, the official guidance from major labs like Quest Diagnostics is straightforward: don’t eat or drink anything except water. Many providers echo this. The safest approach is to skip coffee and tea the morning of your test. If you absolutely need caffeine to function and your appointment is later in the day, a small cup of plain black coffee (no sugar, no cream, no milk) is unlikely to throw off most results. But “unlikely” and “approved” aren’t the same thing, and some tests are more sensitive than others.
Drinks That Will Break Your Fast
- Coffee with cream, sugar, or flavored syrup. Even a splash of milk adds fat and protein that trigger a metabolic response.
- Juice of any kind. Fruit juice is essentially concentrated sugar. Fructose in particular has been shown to raise postprandial triglyceride levels.
- Soda and energy drinks. Both regular and many “zero calorie” versions contain ingredients that can interfere with metabolic markers.
- Smoothies, protein shakes, and milk. These are meals in liquid form.
- Alcohol. It affects liver function, blood sugar regulation, and triglyceride metabolism for hours after consumption.
Flavored Water, Lemon Water, and Sparkling Water
Skip the lemon slice. UPMC’s guidelines specifically warn against adding lemon or other flavorings to your water before a blood test. Even small amounts of citrus contain natural sugars and acids that could subtly affect results. The same goes for flavored water products, which often contain sweeteners or citric acid.
Sparkling water is a gray area. Plain seltzer (just water and carbonation, with no sodium, sweeteners, or flavors) is unlikely to cause problems for most tests. But there is preliminary evidence suggesting that carbonation may influence how the body absorbs and processes glucose, because CO₂ absorbed through the stomach lining triggers an alkalinization process in red blood cells that can speed up glucose uptake. This research is still theoretical and hasn’t been confirmed in rigorous human trials, but if you’re getting a glucose test, still water is the cleaner choice.
Gum, Mints, and Other Surprises
Sugar-free gum and mints are easy to overlook, but they contain small amounts of sweeteners that can trigger digestive enzymes and a minor insulin response. Most lab guidelines lump them in with food. If you’re worried about bad breath for your morning appointment, brush your teeth instead (just don’t swallow the toothpaste).
Medications During Your Fast
Most routine prescription medications can be taken with water during your fasting period. Skipping a dose of blood pressure or thyroid medication, for example, could be more harmful than any minor effect on your lab results. That said, some medications directly affect blood sugar or lipid levels, so confirm with your provider which ones to take and which to hold until after the draw.
Practical Tips for Your Fasting Window
The easiest strategy is to schedule your blood draw first thing in the morning. If your appointment is at 8 a.m., you stop eating by midnight and sleep through most of the fast. Drink a full glass of water when you wake up and another on your way to the lab. Your veins will thank you, and the whole process goes faster.
If a morning appointment isn’t possible and you’re fasting during waking hours, set a timer on your phone for when your eating window closes. It’s surprisingly easy to mindlessly grab a handful of nuts or sip a latte and forget you’re supposed to be fasting. Keep a water bottle nearby as a reminder that water is your only option until the draw is done.