Can I drink water before a fasting blood test? Yes. Plain water is allowed, and it actually helps. MedlinePlus, the U.S. National Library of Medicine’s health site, says that while fasting “you can drink plain water.” Quest Diagnostics explains why water helps: “Water hydrates your veins. Hydrated veins are easier to find.” What you cannot have is anything else: no coffee, juice, soda, flavored water, gum or food.
Below is what counts as breaking a fast, how long fasting usually lasts, which tests need it, and what to do if you slip. Your own doctor’s instructions always come first, because some tests have special rules.
Can you drink water while fasting for blood work?
Yes, and you should not skip it. Quest Diagnostics defines fasting this way: “Fasting before a blood draw means you don’t eat or drink anything except water.” Going without water can leave you a little dehydrated, which makes veins harder to find and the draw slower.
The water has to be plain. MedlinePlus says to avoid lemon-flavored water and flavored carbonated water, which may contain sugar or artificial sweeteners. Plain sparkling water with nothing added is not mentioned specifically, so if your test is for blood sugar, we would stick to still tap or bottled water to be safe.
What breaks a fast before a blood test?
| Allowed | Not allowed |
|---|---|
| Plain water, still | Coffee, including black coffee |
| Your usual medicines, unless your doctor says otherwise | Tea, juice, soda, energy drinks |
| Flavored or sweetened water | |
| Chewing gum and mints | |
| Smoking | |
| Exercise |
MedlinePlus says “juice, coffee, soda, and other drinks can affect your results,” and lists gum, smoking and exercise as things not to do while fasting. Black coffee surprises people the most: even without sugar or milk, it is not water, so leave it until after the draw.
How long do you have to fast before a blood test?
Usually 8 to 12 hours. MedlinePlus says, “You usually need to fast for 8 to 12 hours before a test,” and Quest says fasting “typically lasts eight hours,” with your doctor’s instructions taking priority. That is why fasting tests are usually booked first thing in the morning: most of the fast happens while you sleep.
Water is allowed for the whole fasting period, including the morning of the test.
Which blood tests need fasting?
According to MedlinePlus and Quest, the tests that usually require fasting are:
- Blood glucose (blood sugar) tests, though “not all blood glucose tests require fasting,” per MedlinePlus
- Cholesterol and lipid panels, including triglycerides and HDL
- The basic metabolic panel (BMP)
- Sometimes liver function tests and kidney function panels
Plenty of routine blood tests do not need fasting at all. If your paperwork does not say “fasting,” ask before you skip breakfast.
Should you take your medicines before a fasting blood test?
Usually yes, with water. Quest says, “Unless your doctor says otherwise, take your usual medications.” MedlinePlus adds a firm warning: “don’t stop taking your medicines unless your provider tells you to stop.” Some tests do need a medicine paused, so ask when the test is ordered, and mention any vitamins or supplements too.
What if you accidentally ate or drank something?
Tell the person drawing your blood or your provider. MedlinePlus says you may need to reschedule so you can fast again and get accurate results. A test done after eating can show high blood sugar or triglycerides that are not real, which can lead to wrong conclusions.
How much water should you drink before blood work?
No official amount applies to everyone. Our practical advice: drink normally the evening before, and have a glass or two of plain water in the morning before you leave. That keeps you hydrated without making you need the bathroom in the waiting room. If you have a condition that limits fluids, follow your doctor’s advice instead. For everyday hydration, our guide on how much water you should drink per day covers the general recommendations.
Frequently asked questions
Can I drink water before a fasting blood test?
Yes. MedlinePlus says you can drink plain water while fasting, and Quest Diagnostics says hydrated veins are easier to find.
Can I drink black coffee before a fasting blood test?
No. MedlinePlus says juice, coffee, soda and other drinks can affect your results. Only plain water is allowed.
Can I drink sparkling water while fasting for blood work?
Avoid flavored or sweetened sparkling water. MedlinePlus specifically warns against flavored carbonated water. Plain still water is the safe choice.
Does drinking water affect blood test results?
Plain water does not break a fast for routine fasting tests, and it makes the blood draw easier. Other drinks can change results.
Can I chew gum before a fasting blood test?
No. MedlinePlus lists gum, smoking and exercise among the things not to do while fasting.
How many hours do I need to fast?
Usually 8 to 12 hours, per MedlinePlus. Follow the exact time your provider gives you.
This article is general information, not medical advice. Follow the instructions from the provider who ordered your test.
Sources
- MedlinePlus, U.S. National Library of Medicine, Fasting for a Blood Test.
- Quest Diagnostics, Fasting for lab tests.
