Can an infection raise blood sugar?

Two things happen at once

Inflammatory cytokines induce insulin resistance. The immune response deliberately makes cells less responsive to insulin, keeping glucose available in the bloodstream.

The stress response releases stored glucose. Cortisol and adrenaline tell the liver to produce more.

Together these can push glucose up sharply — and, crucially, they do it even when you are eating less than usual. People often assume that eating almost nothing while ill should bring numbers down, and are alarmed when the opposite happens.

Why this is the situation that matters most

Illness is the most common precipitant of diabetic ketoacidosis. The combination is what does it: glucose rising, insulin needs increasing, appetite gone, and often vomiting or diarrhea causing fluid loss on top.

That is why sick-day rules exist, and why they are worth knowing before you need them.

Sick-day rules

Your care team's specific version takes precedence, but broadly:

• Keep taking your insulin, even if you are eating little. This is the single most important rule and the one most often broken. Insulin needs usually rise during illness.
• Test glucose more often — every 2 to 4 hours
• Check ketones if you are above 240 mg/dL (13.3 mmol/L) and use insulin
• Keep fluids going. Sips frequently if you are nauseated
• If you cannot eat, take carbohydrate as fluid — this is one situation where a sugary drink can be appropriate, if your glucose is not high
• Know your threshold for calling. Have it written down before you are ill

Get help if

• You cannot keep fluids down
• Moderate or large ketones
• Deep or rapid breathing, or fruity-smelling breath
• Confusion or unusual drowsiness
• Glucose staying above 300 mg/dL (16.7 mmol/L) despite correction
• Any illness in a young child or older adult with diabetes that is not clearly improving

It works the other way too

High glucose also makes infections more likely and slower to resolve — it impairs the white blood cells that fight them and provides a hospitable environment for yeast and bacteria. Recurrent thrush, urinary infections or slow-healing wounds are a reason to check glucose, not just to treat the infection.

Metformin is often paused during acute illness with dehydration or vomiting. If you take it, ask what your plan should be — before you are unwell.