What happens to medications on sick days?

A feverish infection puts the body on alert, pushing up counter-regulatory hormones like cortisol, adrenaline, and glucagon. They blunt how tissues answer insulin and speed the liver’s sugar production. So being sick carries blood sugar up even without eating. Another force works against this. Vomiting and diarrhea take both food and fluid, pulling the picture the other way.

The real fragile point: running out of fluid. As fluid drops, kidney blood flow falls and the kidney clears substances from the blood more slowly. Drugs leaving mainly through the kidney feel that slowdown directly. Metformin is the best-known example: what normally exits fine builds up in the blood when that exit narrows, straining the lactic acid balance.

SGLT-2 inhibitors follow a separate route. Because this family sends sugar out in the urine, it already raises fluid loss; the illness’s loss stacks on top. There’s a second effect too. It shifts the insulin and glucagon balance, the body turns to fat for fuel, and ketone production speeds up. The resulting acid picture can develop even when the reading doesn’t look high; the literature calls it “euglycemic ketoacidosis”.

Both pictures are rare. Rare doesn’t mean unimportant when conditions stack up. The reason for that fragility varies from family to family; each family’s weak point sits elsewhere.

• Metformin: its exit runs almost wholly through the kidney, which fluid loss narrows first.
• SGLT-2 inhibitor: deepens fluid loss and makes ketone production easier.
• Sulfonylurea: keeps prompting the pancreas; without food, low sugar grows likelier.
• GLP-1 receptor agonist: slows stomach emptying, deepens nausea and loss of appetite.
• Some blood pressure drugs and water pills: push fluid loss and kidney load in the same direction.
• Insulin: reversed on this list; the need doesn’t vanish.

Insulin really does sit elsewhere on this list. During an infection the body’s insulin need doesn’t shrink; most often it grows. Insulin dropping out altogether because the plate stayed empty opens a second door on top of the first problem: the body shifts to burning fat and ketones pile up. So being unable to eat doesn’t zero the insulin need by itself.

Here’s the sum of all this. What happens on a sick day depends on the drug family, kidney status, whether food stays down, and how many days the illness runs. A single general sentence can’t cover these four variables at once. So sick day rules are a ready-made plan the person builds with the care team while healthy. New decisions come hard when fever climbs and sleep is broken. A text written beforehand moves the decision to a healthy day. Some keep that paper in the nightstand drawer, by the thermometer.