When your blood sugar is high, the first steps are to drink water, check for ketones if you’re above 250 mg/dL, and take your prescribed insulin if you have a correction dose. Most mild to moderate spikes come down within a few hours with the right response, but readings above 300 mg/dL that won’t budge need emergency attention.
What you should do depends on how high the number is, whether you have type 1 or type 2 diabetes, and what symptoms you’re experiencing. Here’s a practical breakdown.
Start With Water and Recheck
High blood sugar pulls water out of your cells and into your bloodstream, then your kidneys try to flush the excess glucose through urine. This cycle dehydrates you quickly, and dehydration makes the spike worse. Drinking water helps your kidneys clear glucose and keeps your blood volume stable. Aim for a full glass right away and keep sipping steadily over the next hour or two.
Recheck your blood sugar about 30 to 60 minutes after you take any corrective action. If your number is dropping, you’re on the right track. If it’s staying flat or climbing, you may need additional steps.
Take Your Correction Insulin If Prescribed
If you use rapid-acting insulin and your doctor has given you a correction factor, this is when to use it. A correction factor tells you how much one unit of insulin will lower your blood sugar. For many people, one unit drops blood glucose by roughly 40 to 50 mg/dL, but this varies widely, anywhere from 15 to 100 mg/dL per unit depending on your body and your total daily insulin dose.
The basic math: subtract your target blood sugar from your current reading, then divide by your correction factor. If your blood sugar is 250 mg/dL, your target is 120, and your correction factor is 50, you’d take about 2.5 units. Never guess at a correction dose if you haven’t been given one by your care team. Too much insulin creates a dangerous low, which is a more immediately life-threatening problem than a high.
Check for Ketones Above 250 mg/dL
When your body can’t use glucose for energy (usually because there isn’t enough insulin), it starts burning fat instead. That process produces ketones, acidic byproducts that build up in your blood. This is especially important for people with type 1 diabetes, but it can happen in type 2 as well.
Many glucose meters prompt you to test for ketones when your reading crosses 250 mg/dL, and that’s a good threshold to follow. A blood ketone reading below 0.3 mmol/L is normal. Once ketones reach 1.5 mmol/L or higher, you should contact your doctor promptly. At 3.0 mmol/L, you need urgent medical attention because you’re approaching or already in diabetic ketoacidosis.
When Exercise Helps and When It Doesn’t
Physical activity is one of the most effective non-medication tools for bringing blood sugar down. A single session of aerobic exercise, even a 15- to 20-minute walk, improves your body’s ability to use insulin for 24 to 72 hours afterward, depending on how long and how hard you go. If your blood sugar is moderately elevated and you feel okay, light to moderate movement can make a real difference.
There’s an important exception. If your blood sugar is 250 mg/dL or higher, check for ketones before exercising. The American Diabetes Association recommends postponing all exercise if blood ketones are at or above 1.5 mmol/L, or if a urine test shows moderate to large ketones. At readings of 350 mg/dL or above, ketone testing is essential before any physical activity. Exercising with elevated ketones can actually push both your blood sugar and ketone levels higher, increasing your risk of ketoacidosis.
If ketones are negative and you feel well, gentle activity like walking is generally safe even at higher readings. Avoid intense exercise until your numbers come down.
Look at What Caused the Spike
While you’re waiting for your blood sugar to drop, think about what pushed it up. Common culprits include eating more carbohydrates than your insulin or medication covered, missing a dose, stress, illness, poor sleep, or a medication change. Identifying the trigger helps you prevent the next spike.
Illness and infection deserve special attention. When you’re sick, your body releases stress hormones that raise blood sugar even if you’re not eating much. If you’re dealing with a cold, flu, or any infection, check your blood sugar more frequently and follow any sick-day plan your doctor has outlined. Dehydration from vomiting or fever compounds the problem.
Know the Emergency Warning Signs
Most high blood sugar episodes resolve with hydration, correction insulin, and time. But two complications require an emergency room visit: diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS).
Diabetic Ketoacidosis
DKA is most common in type 1 diabetes and develops when ketones build up to dangerous levels. Warning signs include fast, deep breathing, fruity-smelling breath, nausea and vomiting, stomach pain, extreme fatigue, and dry mouth. The CDC recommends going to the emergency room or calling 911 if your blood sugar stays at or above 300 mg/dL, your breath smells fruity, you’re vomiting and can’t keep fluids down, or you’re having trouble breathing. DKA can escalate within hours.
Hyperosmolar Hyperglycemic State
HHS is more common in type 2 diabetes and involves extremely high blood sugar, typically 600 mg/dL and above, sometimes reaching over 1,000. Unlike DKA, significant ketone buildup usually isn’t present. The hallmark of HHS is severe dehydration leading to neurological changes: confusion, vision problems, extreme drowsiness, or inability to stay alert. You may notice dramatically increased thirst and urination in the days leading up to it, followed by weakness and lethargy. HHS develops more slowly than DKA, often over days or weeks, but it’s equally dangerous. If someone with diabetes becomes confused or difficult to rouse, call 911 immediately.
After the Spike Comes Down
Once your blood sugar returns to your target range, don’t stop paying attention. Check again in another one to two hours to make sure the drop holds steady and doesn’t overshoot into a low, especially if you took correction insulin. Eating a small, balanced meal with protein and fiber can help stabilize things.
If you’re seeing frequent spikes above 250 mg/dL, that’s a sign your overall management plan may need adjusting. Track when the highs happen, what you ate beforehand, and how much insulin or medication you took. That pattern gives your care team specific data to work with rather than a vague report of “my sugar keeps going high.” Persistent hyperglycemia causes cumulative damage to blood vessels, nerves, and organs, so repeated spikes are worth addressing even when each individual episode resolves on its own.