Short answer: Illness often pushes blood sugar up, because insulin works less well in the body's cells when you are sick, even at usual doses. But eating less or vomiting can push it down. Insulin labels say doses may need to change during acute illness, under medical supervision. MedlinePlus describes a sick-day plan of more frequent checks, fluids, ketone tests in type 1 diabetes, and clear points to call for help.
MedlinePlus says that when you are sick, insulin does not work as well in your cells and blood sugar can be higher, even with your normal doses. Even a minor cold can make diabetes harder to control. Not wanting to eat or drink can, perhaps surprisingly, also lead to higher sugar.
It can go the other way too. The NIDDK lists being sick among the causes of low blood glucose, because you may not be able to eat as much or keep food down. That two-way pull is why sick-day plans lean on frequent checks rather than guesses.
The insulin labels say this in their own terms. Lantus, Humalog, NovoLog and Humulin N (section 2) say dose changes may be needed "during acute illness," and the Lantus label adds that changes should only be made under medical supervision with glucose monitoring. The Humalog label (section 17) says patients should be taught how to handle "intercurrent conditions (illness, stress, or emotional disturbances)."
MedlinePlus suggests agreeing on a plan with your provider before you get sick. The typical plan it describes:
For lows under 70 mg/dL it describes the "rule of 15": 15 grams of fast-acting carbohydrate, a recheck after 15 minutes, and repeat until back to normal.
| Sign | Threshold |
|---|---|
| High blood sugar | Above 240 mg/dL for more than 1 day |
| Ketones | Moderate to large on a urine test |
| Vomiting or diarrhea | More than 4 hours |
| Fever | 100 °F (37.7 °C) or higher |
| Other | Severe or chest pain, trouble moving arms or legs, vision, speech or balance problems, confusion |
MedlinePlus adds that if you cannot keep food or liquids down after taking insulin, the next step is the emergency room for IV fluids. Ketones matter because they are the early sign of diabetic ketoacidosis.
Some diabetes medicines have their own sick-day rules. SGLT2 medicines such as Jardiance and Farxiga carry a ketoacidosis warning; see SGLT2 medicines before surgery for what their labels say about ketoacidosis and fasting.
MedlinePlus asks you to write down each reading, the time and every medicine taken. Dose Track logs doses with their times and lets you add notes, which gives the provider the full picture when you call.
Read more on Dose Track: Diabetic ketoacidosis · Missed a Lantus dose · Blood sugar · Hypoglycemia (low blood sugar)