Plain water is allowed during a 3-hour glucose tolerance test. The World Health Organization’s own preparation guidelines state that during the overnight fast preceding the test, “water may be drunk,” and that permission extends through the test itself. Most labs and clinicians actively encourage small sips of water while you sit through the three hours of blood draws. Water does not contain calories, sugar, or other nutrients that would interfere with the glucose measurements the test is designed to capture. What matters is that you drink only plain water and avoid anything else that could trigger an insulin or blood sugar response.
Why Water Does Not Skew Your Results
The 3-hour oral glucose tolerance test (OGTT) works by giving you a measured dose of glucose, usually 100 grams for the gestational diabetes version or 75 grams for the standard version, and then tracking how your body handles that sugar over time through a series of blood draws. The concern with consuming anything during the test is that it could independently raise or lower your blood sugar and muddy the picture. Plain water has no glucose, no calories, and no sweeteners, so it has nothing to contribute to that equation.
A small study examining fasting blood glucose levels in people who drank water versus those who did not found that the water-drinking group actually had slightly lower average readings, about a 4% decrease, compared with those who abstained from water entirely. The researchers found a mean fasting glucose of roughly 79 mg/dL with water and about 82 mg/dL without it. If anything, mild dehydration may nudge blood sugar readings slightly higher, not lower.
Research on dehydration and glucose metabolism bears this out more directly. In a study of insulin-deficient subjects, dehydration increased plasma glucose concentrations compared to a normally hydrated control condition, while fasting alone actually reduced glucose levels. The mechanism is straightforward: when you are dehydrated, your blood volume drops, concentrating the glucose already circulating. That means skipping water during a long test could paradoxically make your numbers look worse, not better.
What You Should Not Drink or Eat
The permission for water comes with an important boundary: it is only plain water. During the test, anything that has calories, sugar, artificial sweeteners, or even strong flavors is off limits. That includes coffee, tea, juice, soda, flavored water, and sports drinks. Even diet beverages sweetened with non-caloric sweeteners are not safe. Research has shown that oral exposure to sucralose, a common zero-calorie sweetener, can trigger a measurable insulin spike within two minutes of contact with the mouth in a subset of people, particularly those who are overweight or obese. That early insulin release, sometimes called a cephalic phase insulin response, could alter how your body processes the glucose drink and throw off the results.
Food of any kind is also prohibited. You should not chew gum, eat candy, or have a snack brought from home. The test depends on the glucose solution being the only source of sugar entering your system during the entire monitoring window.
Preparing in the Days Before the Test
The rules around the test itself are simpler than most people expect, but the preparation leading up to it matters more than many realize. For at least three days before the OGTT, you should eat a normal or even slightly carbohydrate-heavy diet, aiming for at least 150 grams of carbohydrates per day spread across three meals. The meal the evening before the test should include at least 50 grams of carbohydrates. This is not a time to start a low-carb diet, go on a cleanse, or skip meals. Restricting carbohydrates in the days before the test can actually impair your glucose tolerance temporarily, making your results look abnormally high even if you would normally process sugar without issue.
After that final evening meal, you fast for 10 to 16 hours overnight. During that fast, water is the one thing you are encouraged to drink. Some people worry that even a few sips of water might “break the fast.” It does not. A fast for the purposes of this test means no caloric intake. Water passes through your stomach without triggering the digestive processes that would affect blood sugar or insulin levels.
Smoking, Stress, and Other Things That Actually Do Interfere
While water is harmless during the test, several other common behaviors can genuinely distort results. Smoking is one of the most studied. A trial comparing glucose tolerance in smokers who smoked during the test versus the same smokers on a non-smoking day found that smoking acutely impaired glucose tolerance, with the glucose area under the curve rising from about 21.8 to 25.5 mmol/L. Smoking also raised insulin levels while decreasing insulin sensitivity. Even in non-smokers exposed to cigarette smoke during the test, glucose readings climbed modestly. The takeaway: do not smoke before or during the OGTT.
Stress is another factor people rarely think about, but the physiology is clear. Psychological stress triggers the release of cortisol and adrenaline, both of which push blood sugar up by promoting glucose production in the liver and increasing insulin resistance. Sitting in a medical office for three hours with repeated needle sticks is inherently stressful for some people, and that stress alone can elevate readings. There is no perfect solution for test anxiety, but knowing that stress can affect results is useful. If your numbers come back borderline and you were unusually anxious during the test, that context is worth sharing with your provider.
Physical activity also matters. The standard guidance is to remain seated and relatively still during the three-hour window. Walking around, climbing stairs, or fidgeting excessively can increase glucose uptake into your muscles and artificially lower your readings. On the flip side, unusually intense exercise in the day or two before the test can temporarily improve insulin sensitivity and mask a genuine issue. The goal is to show up having lived your normal life for the previous few days.
What the Three Hours Actually Look Like
If you have never done a 3-hour OGTT, the logistics are straightforward but the experience can be unpleasant. You arrive fasting, and a technician draws your baseline blood sample. Then you drink the glucose solution, a very sweet, syrupy liquid that typically comes in orange or fruit punch flavor. You have a limited window, usually five minutes, to finish it. After that, the clock starts.
Blood is drawn at the one-hour, two-hour, and three-hour marks. Between draws, you sit in the waiting room. You cannot eat, you cannot leave the facility, and you should not exercise. You can drink small amounts of plain water, read, scroll your phone, or do anything else sedentary. Many clinics will let you bring a book or headphones.
The test is most commonly ordered during pregnancy as a follow-up to an abnormal one-hour glucose screening. The one-hour screening uses a 50-gram glucose load and a single blood draw one hour later. If that result is elevated, the 3-hour version with a 100-gram load and four blood draws gives a more definitive answer about gestational diabetes. Outside of pregnancy, the standard OGTT typically uses a 75-gram load and runs for two hours rather than three, though the preparation and water rules are identical.
Dealing with Nausea and Other Side Effects
The glucose solution is intensely sweet, and drinking it on an empty stomach after an overnight fast makes many people feel sick. Nausea is by far the most common complaint, but dizziness, weakness, and even diarrhea are all reported. A study of patients who had undergone bariatric surgery found that nearly two-thirds experienced adverse effects during the OGTT, including nausea in about 38%, dizziness in roughly 31%, weakness in 26%, and diarrhea in about 23%. Post-bariatric patients are an extreme case because their altered anatomy processes the glucose solution differently, but even people with a normal gastrointestinal tract frequently feel queasy.
This is one reason water is so helpful during the test. Small sips can settle your stomach, keep your mouth from feeling uncomfortably dry, and help you tolerate the wait. Drinking a large volume of water all at once is not ideal, because a very cold or very large drink can alter gastric motility. Research has shown that cold water slows the frequency of gastric contractions compared to warm water, and the rate of gastric contractions is linked to how much energy reaches the small intestine. A few moderate sips of room-temperature or cool water is the sweet spot: enough to stay comfortable without introducing a variable that could change how quickly the glucose solution moves through your system.
If you vomit during the test, tell the staff immediately. In most cases, the test will need to be rescheduled because the glucose dose is no longer reliable. This is uncommon but not rare, and there is no penalty or extra cost in most clinical settings for needing a redo.
How Much Water Is Reasonable
Guidelines do not typically specify an exact amount of water you can drink during the test. The general principle is “sip as needed.” You are not trying to hydrate aggressively or drink multiple glasses. A few ounces here and there over three hours is fine. Some labs will have a water cooler in the waiting area for exactly this purpose.
There is one edge case worth knowing about. If you drink a very large volume of water, say 32 ounces or more in a short window, you could theoretically dilute your blood slightly and affect lab values across the board, not just glucose. This is the same reason blood donation centers ask you to hydrate beforehand but not to chug water right before. For a glucose test, no one is likely to drink that much while sitting in a waiting room, so it is more of a theoretical concern than a practical one. Just drink normally when you are thirsty.
What Happens if You Accidentally Break the Rules
People sometimes slip up. Maybe you absentmindedly took a sip of your partner’s coffee, chewed a piece of gum, or forgot you were not supposed to eat and had a handful of crackers at 6 a.m. before remembering. If this happens, tell the lab technician before the test begins. In most cases, the test will be rescheduled. Running the test after a protocol violation wastes your time and the lab’s resources, and a borderline result would be uninterpretable. It is better to come back another day than to sit through three hours of blood draws for results no one can trust.
The same applies if you realize you did not eat enough carbohydrates in the days leading up to the test. As noted earlier, carbohydrate restriction before the OGTT can produce falsely elevated glucose readings. If you have been following a very low-carb or ketogenic diet, let your provider know. They may recommend a longer period of carbohydrate reintroduction before scheduling the test, or they may use an alternative diagnostic method entirely.
After the Test
Once the final blood draw is done, you are free to eat and drink whatever you want. Most clinics will suggest you have a snack or meal ready because you will have been fasting for many hours by that point and your blood sugar may be low. Some people, especially those whose bodies produce a large insulin response to the glucose load, can experience a reactive blood sugar drop in the hours after the test. This feels like shakiness, lightheadedness, or sudden hunger, and eating something with protein and complex carbohydrates resolves it quickly.
Research into what happens during prolonged glucose tolerance testing has identified a pattern called reactive hypoglycemia, where blood sugar drops significantly below baseline after the glucose is cleared. One study found that patients who experienced genuine reactive hypoglycemia tended to have a steep initial glucose spike, with a change of about 74 mg/dL in the first hour compared to 34 mg/dL in those without the condition. If you routinely feel terrible after eating sugary foods, this pattern might be worth discussing with your doctor independently of the test results.
When Water Temperature Matters
One detail that occasionally comes up in online discussions is whether the temperature of the water you drink during the test matters. The honest answer is that it probably does, but only slightly. A study measuring the effects of water temperature on stomach activity found that very cold water (around 2°C, or 36°F) reduced the frequency of gastric contractions compared to warm water (60°C, or 140°F), and that contraction frequency was linked to energy intake. In theory, ice-cold water could slow how quickly the glucose solution empties from your stomach into the small intestine, where absorption happens. A slower emptying rate might spread the glucose absorption over a longer period and slightly blunt the peak blood sugar reading.
In practice, this effect is small enough that no clinical guideline addresses it. The water available in most waiting rooms is room temperature or cool from a cooler. If you bring a water bottle, you do not need to worry about whether it has been in the fridge. Just avoid extremes: do not bring a thermos of hot water, and do not chug ice water right before a blood draw. Moderate temperature, moderate volume, taken in sips. That is all the thought this deserves.
Caffeine Withdrawal and Morning Medications
If you are a regular coffee or tea drinker, the overnight fast and morning without caffeine can cause a headache or irritability by the time you are sitting in the waiting room. Caffeine withdrawal is not going to change your glucose results, but it can make an already uncomfortable experience feel worse. There is no great solution here. You cannot have coffee during the test, and caffeinated beverages are off limits even if they are black and sugar-free, because caffeine itself may affect insulin sensitivity in some people. Knowing the headache is coming and planning for it, perhaps by tapering caffeine slightly in the days before, can help.
As for medications, the rules vary. Some drugs, including corticosteroids, beta-blockers, and certain diuretics, can affect blood glucose. Your ordering provider should tell you which medications to take and which to hold on the morning of the test. If they did not bring it up, call and ask. Prenatal vitamins are generally fine to take with a sip of water the morning of the test, but confirm with your clinic. The point is that “nothing by mouth except water” does not necessarily extend to prescribed medications, and you should not skip essential drugs without explicit instructions to do so.