Can You Drink Water if You Are NPO for Blood Work? The Hydration Rule Explained

October 1, 2025 •

4 min read

For most fasting blood tests, drinking plain water is not only allowed but encouraged by medical professionals. Understanding the specific rules regarding what you can and cannot consume is crucial for ensuring accurate results when you are instructed to be NPO for blood work.

Quick Summary

It is generally permissible to drink plain water when asked to fast for blood work, as it does not affect most test results and helps with the blood draw. However, consuming other beverages or food can skew results, potentially requiring a retest. Always follow your doctor's specific guidelines.

Key Points

Plain Water is Allowed: For most fasting blood tests, plain, still water is the only beverage permitted and is actually encouraged to keep you hydrated.
Stay Hydrated for an Easier Draw: Drinking water helps plump up your veins, making it easier for the phlebotomist to find them and perform the blood draw.
Avoid All Other Liquids: Beverages like coffee, tea, juice, and soda must be avoided, as they can alter glucose and other substance levels in your blood.
No Gum or Smoking: Chewing gum and smoking can stimulate the digestive system and affect test results, so they should also be avoided during the fast.
Confirm with Your Doctor: Always verify with your healthcare provider whether fasting is required for your specific test and for how long, as not all tests require it.
Inform the Lab if You Fail to Fast: If you accidentally eat or drink something other than water, inform the lab technician so they can determine if the test can still be performed or needs to be rescheduled.

What NPO Means in a Medical Context

NPO stands for the Latin phrase nil per os, which translates to 'nothing by mouth'. It is a critical medical instruction used to ensure a patient's stomach is empty for certain procedures, such as surgery or specific diagnostic tests. The strictness of NPO varies, and for most fasting blood tests, the rule has a crucial exception: plain water. Medical providers give these instructions because nutrients and substances from food and most beverages are absorbed into the bloodstream, where they can temporarily alter the levels of various substances being measured. If you do not follow fasting instructions, your test results may be inaccurate, leading to a potential misdiagnosis or the need for a repeat test.

The Crucial Exception: Why Water is Different

Drinking plain, still water is the one thing almost always permitted—and often recommended—before a fasting blood test. This is because pure water contains no calories, sugars, or other compounds that would interfere with common fasting tests, such as glucose or lipid panels. In fact, staying hydrated is highly beneficial for several reasons:

Easier Blood Draw: Dehydration can cause your veins to shrink, making them more difficult for the phlebotomist to locate and draw blood from. Drinking water helps keep your veins full and plump, ensuring a smoother, less painful experience.
Accurate Results: While excessive water intake can potentially dilute electrolytes, a normal amount of water consumption typically has no significant effect on most common fasting lab values. Conversely, being dehydrated can also skew results.
Prevents Dizziness: Fasting can sometimes cause lightheadedness. Staying hydrated helps maintain stable blood pressure and can reduce the risk of feeling faint during or after the blood draw.

What to Avoid During a Fasting Period

When following an NPO order for blood work, it's essential to understand that 'nothing by mouth' means abstaining from everything except plain, still water. Here is a list of items to avoid:

Coffee and Tea: Both caffeinated and decaffeinated versions contain compounds that can affect test results. Caffeine is also a diuretic, which can lead to dehydration and make the blood draw more difficult.
Juice and Soda: These beverages contain sugar, which will cause your blood glucose levels to spike and invalidate a fasting glucose test.
Flavored or Carbonated Water: Added flavorings, even zero-calorie ones, can contain ingredients that may affect your results. Carbonation can also potentially trigger a minimal insulin release.
Alcohol: You should avoid alcohol for at least 24 hours before a fasting blood test, as it can affect a variety of markers, including enzymes, blood sugar, and fat levels.
Gum and Mints: Chewing gum, even sugar-free, stimulates the digestive system and can affect certain test results.
Strenuous Exercise: Avoid heavy physical activity in the 24 hours before your test, as it can influence levels of glucose and enzyme markers.

Common Fasting Blood Tests

Fasting is not required for all blood tests, but it is necessary for those that measure components directly affected by recent food intake. Your doctor or the lab technician will inform you if your specific tests require a fast.

Fasting Blood Tests

Glucose Test: Measures your blood sugar levels to help diagnose or monitor diabetes. Eating before this test will give a falsely high reading.
Lipid Panel: Checks levels of cholesterol and triglycerides. Triglyceride levels are particularly sensitive to recent food intake.
Iron Studies: Measures iron levels, which can be temporarily inflated by food containing iron.

Non-Fasting Blood Tests

Complete Blood Count (CBC): Measures red and white blood cells and platelets.
Thyroid Function Tests: Evaluates thyroid hormone levels.
Hemoglobin A1c: Measures average blood sugar over the past few months and does not require fasting.

A Comparative Look: What to Do and What to Avoid


Action	Permitted for Fasting Blood Work?	Reason
Drinking plain water	Yes	Keeps you hydrated and does not affect test results.
Drinking coffee or tea	No	Can skew glucose levels and cause dehydration.
Drinking juice or soda	No	Contains sugars that drastically alter blood glucose levels.
Chewing gum or mints	No	Stimulates digestion and affects certain blood markers.
Taking prescription medication	Usually Yes	Continue as normal unless specifically instructed otherwise by your doctor.
Exercising	No	Strenuous activity can alter enzyme and glucose levels.
Smoking	No	Can affect test results.

How to Prepare for a Fasting Blood Test

To make your fasting experience as smooth as possible, consider the following steps:

Confirm Fasting Requirements: Always double-check with your healthcare provider whether your specific test requires fasting and for how long.
Schedule for Morning: Most people find it easiest to schedule their blood test first thing in the morning. This way, the majority of the fasting period happens while you're asleep.
Drink Plenty of Water: Stay well-hydrated in the hours leading up to your test by drinking plain, still water. This will make the blood draw easier.
Inform the Lab: If you accidentally consume something other than water, inform the lab technician. They may need to make a note for the doctor or reschedule your test. Being honest is crucial for getting accurate results.
Pack a Snack: Bring a small snack with you to eat immediately after your blood is drawn to restore your energy levels.

Conclusion

In summary, the question of whether you can drink water if you are NPO for blood work has a clear and reassuring answer: yes, you can and should drink plain water. This vital exception to the 'nothing by mouth' rule helps ensure accurate test results by keeping you properly hydrated, which also aids the blood draw process. Forgetting this crucial detail can lead to dehydration, difficult venipuncture, and the need for a retest due to inaccurate readings. By following the proper instructions and sticking to only plain water, you can help ensure a quick, efficient, and accurate blood test. For more information on lab tests and preparation, you can visit the MedlinePlus website.

Frequently Asked Questions

NPO stands for the Latin term nil per os, which means "nothing by mouth". For blood work, this means you must not eat or drink anything, except plain water, for a specific period of time before the test.

Plain water contains no calories or sugars that can interfere with test results, unlike other beverages. Drinking water also keeps you hydrated, which makes the blood draw process easier.

There is no strict limit on the amount of water, but it's important to drink enough to stay hydrated without overdoing it. Some professionals recommend drinking 1-2 glasses of water in the hour before your test.

If you drink coffee or any other prohibited beverage, you should tell the lab technician. Your test may need to be rescheduled, as the caffeine and other compounds in coffee can affect test results.

No, it is best to stick with plain, still water. Some experts recommend avoiding sparkling water because the carbonation can potentially stimulate digestion and affect results.

Common tests that require fasting include glucose tests (for blood sugar) and lipid panels (for cholesterol and triglycerides). Your doctor will specify if fasting is needed for your particular tests.

You should continue to take any prescribed medication with water unless your doctor specifically tells you not to. Always confirm with your healthcare provider beforehand.