When you’re sick with a cold, flu, stomach bug, or infection, your body releases stress hormones that push blood sugar higher, even if you’re not eating much. Sick day rules are a set of practical steps that help you keep blood sugar in a safer range, avoid dangerous complications like diabetic ketoacidosis (DKA), and know when the situation needs medical attention. These rules apply whether you have type 1 or type 2 diabetes.
Why Illness Raises Blood Sugar
Any kind of physical stress, whether it’s the flu, a urinary tract infection, or even a bad tooth abscess, triggers your body to release hormones like cortisol and adrenaline. These hormones tell your liver to dump extra glucose into the bloodstream to fuel your immune response. The result is blood sugar that climbs even when you haven’t eaten a thing. For people with diabetes, this spike can be steep and stubborn because the body can’t produce or respond to enough insulin to counteract it.
Vomiting and diarrhea add a second layer of risk. They cause dehydration, which concentrates glucose in a smaller volume of blood, making readings even higher. At the same time, if you can’t keep food down, blood sugar can swing low, especially if you’re on insulin or certain oral medications. Illness creates a tug-of-war between rising and falling blood sugar, which is exactly why a structured plan matters.
Check Blood Sugar More Often
On a normal day you might check your blood sugar a few times. During illness, you should test every two to four hours, including overnight if you’re feeling particularly unwell. If you use a continuous glucose monitor, keep a close eye on trend arrows and set tighter alert thresholds so you catch rapid changes early.
Blood sugar readings above 240 mg/dL that don’t come down after a correction dose are a warning sign. Readings below 70 mg/dL need immediate treatment with fast-acting carbohydrates, just as they would on any other day, but they’re more likely during illness if you’re unable to eat.
Test for Ketones
Ketone testing is essential during sick days, especially for people with type 1 diabetes. When your body doesn’t have enough insulin to use glucose for energy, it starts breaking down fat instead, producing ketones as a byproduct. Too many ketones make the blood acidic, a condition called diabetic ketoacidosis that can become life-threatening within hours.
You can test ketones with a urine strip or a blood ketone meter. Blood meters are more accurate and give real-time results. Here’s how to interpret the numbers:
- Below 0.6 mmol/L: Normal. No action needed.
- 0.6 to 1.5 mmol/L: Low to moderate risk. Contact your care team for guidance.
- 1.6 to 2.9 mmol/L: High risk of DKA. Go to the emergency room.
- Above 3.0 mmol/L: Very high risk of DKA. Go to the emergency room immediately.
Test for ketones any time your blood sugar stays above 240 mg/dL, if you’re vomiting repeatedly, or if you start feeling confused, excessively thirsty, or short of breath. People with type 2 diabetes on certain medications (more on that below) should also test for ketones during illness, even though DKA is less common in type 2.
Keep Taking Your Insulin
One of the most important sick day rules is also one of the most counterintuitive: never stop taking your insulin, even if you’re not eating. Your body actually needs more insulin during illness because of those stress hormones pushing blood sugar up. Skipping doses is one of the fastest routes to DKA.
If you take long-acting (basal) insulin, continue it at your usual dose. Your rapid-acting or mealtime insulin may need adjusting depending on whether you can eat, and your care team should have given you a plan for this. If you don’t have a written sick day insulin plan, call your provider’s office early in the illness rather than guessing.
Medications to Pause During Illness
While insulin should never be stopped, some other diabetes medications need to be temporarily paused when you’re sick, particularly if you’re dehydrated, vomiting, or not eating.
SGLT2 inhibitors (medications with names ending in “-flozin,” like empagliflozin, dapagliflozin, and canagliflozin) carry an FDA warning about ketoacidosis during illness. These drugs work by flushing excess glucose through your urine, which increases dehydration risk and can trigger a form of DKA that sometimes occurs at normal or only mildly elevated blood sugar levels. Current guidance recommends temporarily stopping these medications during acute illness, prolonged fasting, or any time you can’t eat normally. The FDA label specifically notes that clinicians should consider discontinuing the drug “in clinical situations known to predispose to ketoacidosis, such as prolonged fasting due to acute illness.”
Metformin is another medication that may need pausing during severe illness, especially if you’re dehydrated or at risk of kidney stress, because it can cause a rare but serious buildup of lactic acid under those conditions. Your prescriber should tell you in advance which of your medications to hold and when to restart them.
Eating and Drinking When You Feel Awful
Staying hydrated is the single most helpful thing you can do. Dehydration worsens high blood sugar, thickens the blood, and makes it harder for your kidneys to clear excess glucose. Aim to sip water, broth, or sugar-free drinks consistently throughout the day, even if it’s just a few ounces every 15 to 20 minutes.
If you can’t eat regular meals, try to take in about 50 grams of carbohydrates every four hours to prevent blood sugar from dropping too low. That’s roughly equivalent to one and a half cups of unsweetened applesauce, one and a half cups of fruit juice, six saltine crackers with a small amount of peanut butter, or a cup of regular (not diet) gelatin. Small, frequent amounts are easier to keep down than a full meal.
When blood sugar is running high and you’re still managing to drink, choose sugar-free fluids like water, broth, or diet drinks. When blood sugar is trending low or you can’t eat solid food, switch to drinks that contain some sugar, like juice or regular sports drinks, so you get carbohydrates and fluid at the same time.
Watch Out for Hidden Sugar in OTC Medicines
Many over-the-counter cold and flu remedies contain surprising amounts of sugar. Liquid formulations are the worst offenders. NyQuil, for example, contains high fructose corn syrup among its inactive ingredients, adding up to about 20 grams of carbohydrates per dose. That’s essentially a snack’s worth of sugar on top of whatever you’re eating and drinking. Some liquid medicines also contain up to 10% alcohol, which can lower blood sugar unpredictably.
The problem is that sugar content isn’t required on the drug facts label. It hides in the inactive ingredients section under names like high fructose corn syrup, sucrose, or dextrose. Flavorings, colorings, and pill binders can also contain sugar. When choosing an OTC product, look for sugar-free versions (most major brands make them) or choose pill and capsule forms over liquids.
Decongestants like pseudoephedrine deserve extra caution. They work by constricting blood vessels to relieve sinus pressure, but that same mechanism releases adrenaline-like chemicals throughout the body, which can raise both blood pressure and blood sugar. The label on products like Sudafed includes a diabetes warning for this reason.
Prepare a Sick Day Kit in Advance
Illness usually arrives without warning, so having supplies ready saves you a scramble when you’re feeling terrible. A basic sick day kit should include:
- Ketone testing supplies: Blood ketone strips (check expiration dates) or urine strips.
- Extra glucose testing supplies: Enough strips and lancets for testing every two to four hours for several days.
- Fast-acting glucose: Juice boxes, glucose tablets, or regular soda for treating lows.
- Easy-to-digest carbohydrates: Applesauce, crackers, broth, gelatin, and sports drinks.
- Sugar-free cold and flu medication: Pill forms are generally safer than liquids.
- A written sick day plan: Including which medications to adjust, when to test, and your provider’s after-hours phone number.
Signs That Need Emergency Attention
Most illnesses resolve in a few days with careful management at home. But certain symptoms signal that the situation is becoming dangerous. Blood ketones above 1.5 mmol/L, blood sugar that stays above 240 mg/dL despite correction doses, persistent vomiting that prevents you from keeping fluids down for more than a few hours, or signs of DKA (fruity-smelling breath, deep rapid breathing, confusion, or extreme fatigue) all require immediate medical care. DKA can progress from manageable to critical in just a few hours, so acting quickly on these warning signs is essential.