What's the Best Thing to Drink Before a Blood Test?

December 26, 2025 •

4 min read

According to MedlinePlus, for most blood tests that require fasting, plain water is the only liquid you can consume. Drinking water is the best thing to drink before a blood test to ensure accuracy and a smoother collection process.

Quick Summary

This guide explains why plain water is the recommended beverage before a blood test, detailing its benefits for hydration and accurate results. It also highlights which drinks to avoid and answers frequently asked questions about preparing for blood work.

Key Points

Plain Water is Recommended: For most blood tests, including fasting ones, plain water is the only safe beverage to consume before your appointment.
Enhances Vein Visibility: Proper hydration from water increases blood volume, which makes veins easier for the phlebotomist to find, leading to a smoother blood draw.
Avoids Skewed Results: Unlike coffee, juice, or soda, water doesn't interfere with the chemicals and substances measured in blood tests, ensuring accurate results.
Prevents Dizziness: Staying hydrated helps prevent feelings of lightheadedness or fainting that can sometimes occur after an overnight fast and blood collection.
Timing Matters for Fasting: If a fast is required, schedule your appointment early in the morning so you can sleep through most of the fasting period, and always confirm the duration with your doctor.
Skip Coffee, Tea, and Alcohol: These beverages can either dehydrate you (caffeine) or contain compounds that interfere with test results (sugar, alcohol).

Why Plain Water is the Best Choice

For most blood tests, including those that require fasting, plain water is the only permissible liquid. Staying properly hydrated with water before a blood test is crucial for several reasons. First, it helps increase blood volume, which makes your veins fuller and easier for the phlebotomist to locate, resulting in a quicker, less painful blood draw. Dehydration can cause veins to shrink or flatten, complicating the procedure.

Second, drinking water, unlike other beverages, does not interfere with the chemicals and substances in your blood. Many tests measure specific components at their baseline levels. Introducing sugar, caffeine, or other additives can skew results, leading to inaccurate readings and potentially a misdiagnosis or the need for a retest.

Third, adequate hydration can help prevent feelings of dizziness or lightheadedness, which can sometimes occur after an overnight fast and blood collection.

What to Do if You are Fasting

If your doctor instructs you to fast, which is common for tests like a lipid panel or glucose test, the rule is typically to consume nothing but plain water for 8 to 12 hours prior. It is often recommended to schedule your appointment for the morning so that the majority of your fasting period occurs while you are asleep.

Evening Before: Drink plenty of water the day before to get a head start on hydration.
During the Fast: Continue to drink water as needed. A glass or two of plain water on the morning of your test can be beneficial.
Avoid Additives: Do not add lemon juice, flavoring, or anything else to your water during the fasting period.

Which Drinks to Absolutely Avoid

It is just as important to know what not to drink as what to drink. A single slip-up could mean your blood test results are compromised and you have to repeat the process. Here are the main culprits to avoid:

Coffee and Tea: Both regular and decaffeinated coffee and tea should be avoided. Caffeine is a diuretic, which can lead to dehydration. Furthermore, compounds within these drinks can affect certain test parameters.
Fruit Juice: Contains natural sugars that will affect blood glucose levels, making it unsuitable for a fasting glucose test.
Soda and Sugary Drinks: All types of soda, energy drinks, and other sweetened beverages contain sugar and other additives that will invalidate test results.
Alcohol: Alcohol can significantly alter blood sugar and fat levels and should be avoided for at least 24 hours before a fasting blood test.
Milk: A dairy product that contains protein and sugars, which will interfere with fasting test results.

Comparison Table: Safe vs. Unsafe Drinks Before a Fasting Blood Test


Drink Type	Is It Safe?	Why or Why Not?	Best For Which Tests?
Plain Water	Yes	Hydrates veins, doesn't interfere with chemicals	All tests, especially fasting
Coffee	No	Can cause dehydration, affects blood compounds	Non-fasting tests only
Tea	No	Contains compounds that can affect test results	Non-fasting tests only
Fruit Juice	No	High in sugar, affects glucose levels	Non-fasting tests only
Soda	No	High in sugar and additives	Non-fasting tests only
Alcohol	No	Affects blood sugar and fat levels	All tests, avoid for 24h+

What to Consider for Specific Tests

While plain water is a safe bet, specific tests may have slightly different instructions. Always confirm with your healthcare provider about any special preparation instructions for your particular test.

Fasting Blood Glucose Test: Plain water is your only option. Any other drink will affect your blood sugar levels and lead to an inaccurate reading.
Lipid Panel (Cholesterol and Triglycerides): Fasting is typically required for 10 to 12 hours, with only plain water permitted. Triglyceride levels are particularly sensitive to food and drink intake.
Basic Metabolic Panel: Instructions may vary, but often includes fasting. Plain water is almost always safe.
Comprehensive Blood Count (CBC): Fasting is usually not required for this test, but drinking water to stay hydrated is still a good practice.

Conclusion: Water is Your Best Bet

Ultimately, when preparing for a blood test, especially one that requires fasting, plain water is the only universally recommended beverage. Its ability to properly hydrate your body improves blood flow, making the blood draw smoother for you and the phlebotomist. Consuming anything else risks compromising the accuracy of your results and potentially requiring a repeat test. Always follow your doctor's specific instructions, but remember that plain water is the safest and most effective choice.

Disclaimer: The information in this article is for general informational purposes only and does not constitute medical advice. Consult with your healthcare provider for specific instructions regarding your blood test preparation.

Frequently Asked Questions

Drinking plain water is important because it keeps you hydrated. Staying hydrated increases your blood volume, making your veins fuller and easier for the phlebotomist to locate during the blood draw, which can lead to a less painful experience.

No, you should not drink coffee or tea before a fasting blood test. Caffeine can act as a diuretic and dehydrate you. Furthermore, compounds in both coffee and tea can affect blood test results, even if they are unsweetened.

Even if your blood test doesn't require fasting, drinking plain water is still the best option for hydration. It will help ensure the blood collection process goes smoothly without introducing variables from other drinks that could affect certain parameters.

Most sources indicate that plain, unflavored sparkling water is acceptable during a fast. However, you must avoid any flavored carbonated waters, which may contain sugars or artificial sweeteners that can affect your test results.

No, you should only drink plain water during a fasting period. The lemon flavoring could contain trace amounts of sugar or other substances that might affect the accuracy of your test, especially for glucose measurements.

If you accidentally consume something other than water during a fasting period, you should inform your doctor or the lab technician. They may need to reschedule your test to ensure the results are accurate and reliable.

There is no strict rule, but drinking a glass or two of plain water in the hours leading up to your appointment is a good practice. For fasting tests, it is also wise to stay well-hydrated the day before.