Diabetes Sick Day Rules: How to Manage Blood Sugar When You're Ill

blood sugar illness
Women Injecting insulin for Diabetes

When you have diabetes, getting sick is never just getting sick. A cold, the flu, a stomach bug, an infection -- any illness sets off a chain reaction in your body that can send blood sugar climbing even when you're barely eating. And the decisions you make in the first 24 to 48 hours of an illness can be the difference between an uncomfortable week at home and a trip to the emergency room.

Yet most people with diabetes have never been given a clear sick day plan. This guide fixes that. Here are the sick day rules -- what happens to your blood sugar when you're ill, exactly what to monitor and how often, what to eat and drink when you can't keep normal food down, which medications to keep taking, and the specific warning signs that mean it's time to call for help.

Why Illness Raises Blood Sugar -- Even When You're Not Eating

It seems backwards: you're eating less than usual, maybe barely at all, and your blood sugar is higher than ever. Here's why.

When your body fights an infection or illness, it releases stress hormones -- cortisol, adrenaline, glucagon, and growth hormone. These hormones do two things that work against your diabetes management: they signal the liver to release stored glucose into the bloodstream to fuel the immune response, and they make your cells more resistant to insulin at exactly the moment more insulin is needed. The result is rising blood sugar that has nothing to do with what you've eaten.

This is why the single most dangerous sick day mistake is skipping diabetes medication because you're not eating. Your body needs insulin more during illness, not less. Stopping medication during illness is one of the most common preventable causes of diabetic ketoacidosis (DKA) hospitalizations.

The Sick Day Rules

Rule 1: Keep taking your diabetes medication

Unless your healthcare provider specifically tells you otherwise, continue your insulin or oral diabetes medication even if you're not eating normally. People on insulin may actually need more than usual during illness -- your care team may have given you supplemental dosing instructions for sick days; if not, ask for them at your next appointment, before you need them.

A few important exceptions to discuss with your provider: metformin is often paused during illness involving severe vomiting, diarrhea, or dehydration (it can increase the risk of a rare complication called lactic acidosis when you're dehydrated), and SGLT2 inhibitors (medications like Jardiance, Farxiga, and Invokana) are typically paused during significant illness because they increase DKA risk when you're not eating. This is exactly why you need a personalized sick day plan from your care team -- the general rule is keep taking your medication, but specific drugs have specific exceptions.

Rule 2: Test blood sugar every 2 to 4 hours

Normal testing routines aren't enough during illness. Blood sugar can move fast when stress hormones are involved, and you need to catch a climb early. Check every 2 to 4 hours around the clock -- yes, including setting an alarm overnight if your levels are running high. If you use a CGM, illness is when its continuous data earns its keep, but confirm any reading that doesn't match how you feel with a fingerstick, since dehydration can affect sensor accuracy.

This is also the moment your supplies matter most. Running out of test strips mid-illness is a genuinely dangerous situation -- keep at least a full week of extra strips on hand at all times. A fast, reliable meter like the True Metrix Blood Glucose Meter Kit with 4-second results makes round-the-clock testing less of a burden, and stocking up on True Metrix Test Strips before flu season means you're never counting strips while sick. The Autolet Impression Lancing Device with seven depth settings helps when fingertips get sore from frequent testing, and HTL-Strefa MedLance Plus Safety Lancets give a consistently gentle stick.

Rule 3: Check for ketones when blood sugar runs high

If your blood sugar is above 240 mg/dL on two consecutive checks -- or any time you feel nauseated, are vomiting, or have abdominal pain during illness -- check for ketones. Ketones are acids produced when your body burns fat for fuel because it can't use glucose properly, and rising ketones during illness are the warning sign of developing DKA, a medical emergency.

Urine ketone test strips are inexpensive, available without a prescription, and belong in every diabetes sick day kit -- check ours in the diabetic supplies collection or ask your pharmacy. Moderate or large ketones on a urine strip, or blood ketones above 1.5 mmol/L if you use a blood ketone meter, mean call your healthcare provider immediately. This applies to type 2 diabetes as well as type 1 -- DKA is less common in type 2 but absolutely occurs, particularly during severe illness and for anyone on an SGLT2 inhibitor (where it can even occur with near-normal blood sugar).

Rule 4: Stay hydrated -- aggressively

Dehydration is the accelerant of every sick day problem. High blood sugar itself dehydrates you (your kidneys pull water to flush excess glucose into urine), and vomiting, diarrhea, and fever all compound it. Dehydration then concentrates your blood sugar further and impairs kidney function -- a spiral you want to stay ahead of.

Target roughly one cup (8 oz) of fluid every hour you're awake. What to drink depends on your blood sugar:

  • If blood sugar is high: Water, sugar-free electrolyte drinks, broth, or diluted sugar-free beverages
  • If blood sugar is normal or low and you can't eat: Alternate in fluids with carbohydrate -- regular sports drinks, juice, or regular gelatin -- to give your body glucose alongside your medication

Rule 5: Keep carbohydrate coming in, even if meals won't stay down

If you can't eat normal meals, aim for roughly 50 grams of carbohydrate every 3 to 4 hours in whatever form you can tolerate. Easy options when solid food is off the table: regular (not diet) gelatin, saltine crackers, applesauce, toast, broth-based soup with noodles or rice, frozen fruit pops, regular soda sipped slowly, or sports drinks. This isn't the week for perfect nutrition -- it's about giving your body usable fuel so your medication has something to work with and your body doesn't shift into ketone production.

Rule 6: Rest, and don't try to exercise a high down

On a normal day, a walk helps lower elevated blood sugar. On a sick day with ketones present, exercise does the opposite -- it can push blood sugar and ketones higher. If you're ill and running high with ketones, the answer is fluids, medication, and rest, not activity.

Your Sick Day Kit: Build It Before You Need It

The middle of a stomach bug is the worst possible time to discover you're short on supplies. Build a sick day kit now and check it at the start of every fall:

  • Extra test strips -- at least one week beyond your normal supply
  • A reliable glucose meter with fresh batteries (a backup meter like the Prodigy Pocket at $8.88 is cheap insurance)
  • Extra lancets for frequent testing
  • Urine ketone test strips
  • Extra insulin and insulin syringes or pen needles if you use insulin
  • Sugar-free and regular electrolyte drinks, broth, crackers, regular gelatin
  • Fast-acting glucose (tablets or juice) for lows
  • Fever reducer -- note that some cold medications affect blood sugar or blood pressure; ask your pharmacist for diabetes-friendly options
  • Your care team's phone numbers, including the after-hours line, written down
  • Your personalized sick day medication instructions from your provider

When to Call Your Healthcare Provider

Call your care team the same day if:

  • Blood sugar stays above 240 mg/dL despite taking your medication
  • You have moderate or large ketones in urine, or blood ketones above 1.5 mmol/L
  • You've been vomiting or had diarrhea for more than 6 hours and can't keep fluids down
  • The illness isn't improving after 24 to 48 hours
  • You're unsure whether to adjust or pause any medication

When to Seek Emergency Care

Go to the emergency room or call 911 for:

  • Blood sugar above 400 mg/dL that won't come down, or repeated readings below 60 mg/dL that won't come up
  • Large ketones with nausea, vomiting, or abdominal pain
  • Fruity-smelling breath, deep rapid breathing, or confusion -- classic DKA signs
  • Signs of severe dehydration: dizziness on standing, very dark or minimal urine, dry mouth and eyes
  • Difficulty staying awake or alert

DKA develops over hours, not minutes -- caught early with fluids and insulin it's very treatable, which is exactly why the monitoring rules above matter.

Prevention: Two Things That Shrink Your Sick Day Risk

Vaccinations. People with diabetes face significantly higher risk of complications from flu and pneumonia. The annual flu shot, updated COVID vaccination, and the pneumococcal vaccine (ask your provider if you're due) are the highest-leverage prevention available. September and October are the ideal window for the flu shot -- ahead of the season.

Good baseline control. The better your day-to-day management -- consistent monitoring, A1C in range -- the more buffer you have when illness hits. Our guides to understanding your A1C and choosing the best glucose meter cover the foundations. And since illness can also trigger low blood sugar when eating is disrupted, keep our guide to hypoglycemia and the 15-15 rule handy. If a summer illness has you managing insulin outside the fridge, our insulin storage guide covers keeping it safe.

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Build your sick day kit before flu season arrives:

Disclaimer: This information is intended for educational purposes only and should not replace advice from your healthcare provider. Sick day medication adjustments -- especially for insulin, metformin, and SGLT2 inhibitors -- must be personalized by your care team. Ask your provider for a written sick day plan at your next appointment, and seek emergency care immediately for the warning signs listed above.