Illness and Blood Sugar: How Sickness Affects Glucose

The relationship between illness and blood sugar is one of the most practically important — and most commonly mismanaged — aspects of diabetes self-care. When the body is fighting an infection, fever, or other illness, blood sugar almost always rises, often dramatically, even in people who are eating far less than usual. This counterintuitive response — blood sugar going up despite eating less — surprises people who assume that reduced food intake during illness should lower blood sugar. Understanding why illness raises glucose, even when a person is barely eating, is essential for managing diabetes safely through sick days and preventing the acute emergencies that can arise when blood sugar escalates without appropriate response. This guide covers the physiological mechanisms through which illness affects blood sugar, the specific sick-day management rules that apply to people with diabetes, how different types of illness affect glucose differently, and the clinical thresholds that determine when illness-related blood sugar changes require a medical call or emergency evaluation. For the foundational context of how blood sugar is normally regulated and what role insulin plays in that regulation, see our guide on how the body controls blood sugar.

Why Illness Raises Blood Sugar: The Counter-Regulatory Hormone Response

Illness — whether a respiratory infection, urinary tract infection, gastrointestinal illness, or any other acute health challenge — activates the body’s stress response, which includes a surge of counter-regulatory hormones: cortisol, glucagon, epinephrine (adrenaline), and growth hormone. These hormones collectively raise blood glucose through multiple mechanisms: cortisol promotes gluconeogenesis (the liver making new glucose from amino acids and other non-carbohydrate substrates) and reduces insulin sensitivity in muscle and fat tissue; glucagon directly stimulates hepatic glycogenolysis (release of stored liver glycogen as glucose) and gluconeogenesis; epinephrine adds additional gluconeogenesis and glycogenolysis stimulation while suppressing insulin secretion; growth hormone antagonizes insulin action in peripheral tissues. All of these effects operate independently of food intake — they raise blood glucose by increasing the liver’s glucose output and reducing the body’s ability to use the glucose already present, not by adding glucose from food. This is why illness raises blood sugar even when a person with diabetes is eating practically nothing: the illness-driven counter-regulatory response is producing glucose from internal stores and simultaneously making the body less responsive to whatever insulin is circulating. The magnitude of glucose elevation varies with the severity and type of illness: a mild cold may raise fasting glucose by 30–60 mg/dL above the usual level; a significant infection (pneumonia, pyelonephritis, severe urinary tract infection) can raise glucose by 100–200 mg/dL or more; and in people with limited insulin reserve (Type 1, or Type 2 with significant beta cell failure), major illness can trigger diabetic ketoacidosis (DKA) even in the absence of a prior history of DKA. The immune system itself consumes glucose at higher rates during active infection — immune cells are metabolically active and glucose-hungry during an active inflammatory response — but this increased glucose consumption is overwhelmed by the hepatic glucose output driven by counter-regulatory hormones, producing a net rise in blood glucose rather than a fall.

The Risk of DKA During Illness in Type 1 Diabetes

For people with Type 1 diabetes, illness is the leading precipitant of diabetic ketoacidosis — more common than missed insulin doses in many clinical series. The mechanism is the same counter-regulatory hormone surge: with the already-absent or near-absent insulin production of Type 1, the cortisol and glucagon of illness-related stress drive hepatic glucose production to very high levels while simultaneously increasing free fatty acid mobilization from fat tissue. Without insulin to suppress fatty acid release and ketogenesis, the liver converts these fatty acids into ketone bodies at an accelerating rate, producing the acidosis that defines DKA. A common — and dangerous — misunderstanding among people with Type 1 is that insulin doses should be reduced or omitted when eating less during illness. This is incorrect: the illness-driven counter-regulatory response typically increases insulin requirements above the usual dose, not below it, because insulin is needed both to address the elevated blood glucose produced by counter-regulatory hormones and to suppress the ketogenesis that would otherwise proceed unchecked. The correct sick-day approach for Type 1 is to continue all scheduled insulin (basal and bolus) and potentially add correction doses if glucose exceeds targets — not to reduce or skip insulin because eating is reduced. Our guide on symptoms of Type 1 diabetes covers the clinical picture of DKA — nausea, vomiting, fruity breath, abdominal pain, rapid breathing — and our guide on when blood sugar symptoms need medical attention provides the framework for deciding when illness-related glucose elevation and symptoms require emergency evaluation rather than home management.

How Different Types of Illness Affect Blood Sugar

Not all illness affects blood sugar the same way. Understanding how specific types of sickness tend to influence glucose helps set appropriate expectations and management responses.

Respiratory infections (common cold, flu, pneumonia): The counter-regulatory hormone response to respiratory illness consistently raises blood glucose, with greater elevation in more severe illness. Influenza in particular produces significant cytokine release (the immune signaling molecules that drive the inflammatory response) that promotes insulin resistance through mechanisms separate from the classic cortisol/glucagon axis — making flu-related blood sugar elevation both more pronounced and sometimes more prolonged than other respiratory infections. People with diabetes are strongly recommended to receive annual influenza vaccination in part because of the significant sick-day glucose management challenge that influenza presents, and the elevated risk of severe influenza complications (including pneumonia and DKA) in people with poorly controlled diabetes.

Gastrointestinal illness (vomiting, diarrhea): GI illness creates the most complex blood sugar management challenge because it prevents normal eating and may prevent keeping oral medications or food down, while simultaneously activating the counter-regulatory stress response. The dual risk — hypoglycemia from reduced food intake and inability to use oral medications, and hyperglycemia from illness-stress hormones — means blood glucose can swing unpredictably in either direction. People with Type 1 diabetes using insulin must be particularly vigilant during GI illness: if a bolus insulin dose was taken for a meal that is then vomited, the insulin will lower glucose without the food carbohydrate to counterbalance it — producing severe hypoglycemia. The sick-day strategy for GI illness involves matching insulin doses to whatever carbohydrate is actually being retained (not what was planned), using small amounts of easily tolerated carbohydrate (broth, diluted juice, crackers) frequently rather than full meals, and monitoring glucose every one to two hours. If unable to keep any fluids down for more than four to six hours, seeking medical evaluation for intravenous fluid and glucose administration is typically necessary.

Urinary tract infections: UTIs are among the most common infection-related causes of unexplained glucose elevation in people with diabetes, particularly in women. Because UTI symptoms (painful urination, urinary frequency) can overlap with diabetes symptoms, UTIs are sometimes missed until significant glucose elevation prompts evaluation. Any unexplained significant rise in blood sugar in a person with well-controlled diabetes — particularly in a woman — should prompt consideration of UTI as a precipitant. People with diabetes are more susceptible to UTIs in part because glycosuria (glucose in urine) provides a nutrient-rich environment for bacterial growth in the urinary tract, and because the immune function impairment from chronic hyperglycemia reduces the body’s ability to clear infections before they become significant. Addressing the underlying infection is the most important step in normalizing blood sugar after UTI-related glucose elevation.

Managing Blood Sugar During and After Illness

The core principle of sick-day blood sugar management — for both Type 1 and Type 2 diabetes — is that illness almost always increases the body’s glucose-raising tendency and usually increases insulin requirements (either because insulin is being used or because the illness worsens the insulin resistance that oral medications are countering). This means that the default response to illness-related glucose elevation is more active management — more frequent monitoring, correction doses for Type 1, possible temporary dose increases for some Type 2 oral medications — not passive waiting for the illness to resolve. Once the illness resolves, insulin requirements typically return to pre-illness levels within one to two days — sometimes abruptly — requiring awareness to avoid hypoglycemia from a dose that has become excessive as counter-regulatory hormone levels normalize. After recovery, a sick day should prompt a review of what happened: how much did glucose rise, was the management response adequate, were any symptoms alarming enough to have required earlier medical contact, and does anything need to be adjusted for next time? Keeping a blood sugar log that includes illness episodes with their glucose effects and management responses builds the personal data needed to handle future sick days more confidently. Our guide on home blood sugar monitoring explains how to structure this tracking, and our guide on the A1C test provides context for understanding how a significant sick day may affect the next A1C result and what that means for long-term glucose management. For the full context of how blood sugar control during and after illness connects to long-term health outcomes, our guide on why blood sugar matters for long-term health explains why even temporary periods of elevated blood sugar during illness can have lasting metabolic implications — reinforcing the importance of active management rather than passive tolerance of illness-related glucose elevation. Understanding the connection between insulin resistance and why illness amplifies it provides the physiological basis for why sick-day glucose management requires more active intervention than day-to-day management even when the person feels too unwell to focus on blood sugar numbers. And for the diagnostic context — using A1C alongside daily monitoring to understand how illness episodes have affected overall glucose control — our guide on A1C vs blood glucose: what is the difference provides the complementary framework.

Ketone Monitoring During Illness: What to Check and Why

Ketone monitoring becomes critical during illness for people with Type 1 diabetes and for people with Type 2 who use insulin — because the same counter-regulatory hormone surge that raises blood glucose during illness can also trigger ketone production that, if unchecked, progresses to diabetic ketoacidosis. Ketones are produced when the body burns fat for energy in the absence of adequate insulin to drive glucose into cells; during illness, the insulin-suppressing effect of counter-regulatory hormones combined with the glucose-raising effect creates precisely the conditions for ketone accumulation even if insulin is still being taken. Ketone testing is recommended during illness whenever blood glucose is above 240 mg/dL and should be performed using either urine ketone strips (which detect acetoacetate) or a blood ketone meter (which measures beta-hydroxybutyrate, the primary circulating ketone and a more accurate real-time measure). Urine ketone results are interpreted as negative, trace, small, moderate, or large. Any result of moderate or large ketones warrants immediate medical contact — this indicates significant ketone accumulation that can rapidly progress to DKA if not addressed with additional insulin and fluid. Trace or small ketones with glucose above 250 mg/dL also warrant contacting a healthcare provider for guidance on correction dosing and monitoring frequency. Blood ketone meters are more accurate and more immediately actionable: a blood beta-hydroxybutyrate below 0.6 mmol/L is normal; 0.6–1.5 mmol/L indicates elevated ketones requiring more frequent monitoring and correction doses; above 1.5 mmol/L is concerning and requires medical contact; above 3.0 mmol/L indicates significant DKA risk and requires emergency evaluation. People with Type 1 diabetes should have ketone testing supplies (either urine strips or blood ketone meter) accessible at home and should test at the start of any illness that is producing blood glucose above 240 mg/dL, repeating every four hours until ketones are negative and glucose is within target. The presence of nausea or vomiting alongside elevated ketones — even at levels not yet meeting the threshold for emergency evaluation — should prompt earlier medical contact, because vomiting prevents fluid replacement and oral correction and indicates a higher risk of rapid DKA progression. Our guide on symptoms of Type 1 diabetes covers the full DKA symptom picture that indicates when emergency evaluation is needed, and our guide on when blood sugar symptoms need medical attention provides the specific clinical decision framework for deciding between home management and emergency evaluation during illness.

Medications That Require Special Attention During Illness

Several diabetes medications require specific management adjustments during illness that go beyond simply continuing the standard regimen. Understanding which medications need modification — and why — is an important component of sick-day diabetes management that is often inadequately covered in standard diabetes education.

Metformin: Metformin is generally held during significant illness — particularly when there is risk of dehydration, reduced fluid intake, kidney stress, or possible contrast agent administration for imaging. The reason is the small but serious risk of lactic acidosis: metformin accumulates to higher concentrations when kidney function is transiently reduced (as occurs with significant dehydration or acute illness) and can contribute to lactic acid buildup in the rare circumstances when tissue oxygenation is also impaired. Most diabetes care guidelines recommend withholding metformin during any illness significant enough to cause significant vomiting, diarrhea, or reduced fluid intake, and resuming it once the person is eating and drinking normally and kidney function has returned to baseline. This is a decision to discuss with a healthcare provider in advance — ideally having a sick-day plan that specifies when to hold metformin — rather than making ad hoc during the illness itself.

SGLT-2 inhibitors (empagliflozin, dapagliflozin, canagliflozin): SGLT-2 inhibitors carry a recognized risk of diabetic ketoacidosis — including euglycemic DKA, where glucose is not dramatically elevated — that is increased during illness. The mechanism involves reduced carbohydrate intake (which reduces insulin secretion and removes the glucose substrate that normally suppresses ketone production) combined with the direct metabolic effects of SGLT-2 inhibition on fatty acid oxidation. Current guidelines recommend holding SGLT-2 inhibitors during significant illness, surgical procedures, or prolonged fasting, and resuming them once the illness has resolved and normal eating has been established for at least 24–48 hours. People on these medications who develop symptoms of DKA (nausea, vomiting, abdominal pain, lethargy, fruity breath) should seek emergency evaluation even if their glucose appears relatively normal — euglycemic DKA is a specific risk with SGLT-2 inhibitor use that can be missed if evaluation focuses only on glucose level.

Insulin: As noted above, insulin requirements almost always increase during significant illness. The sick-day plan for an insulin user should include guidance from their care provider on how to adjust both basal and correction doses during illness — typically with explicit permission to use more correction insulin more frequently, and with specific glucose thresholds that trigger a call to the provider or emergency services. People who have not received a sick-day plan from their diabetes care provider should request one at their next visit — having this guidance established before illness occurs is far more practical than trying to reach a provider during an acute illness to establish a management plan from scratch.

How Illness Affects A1C and Long-Term Glucose Metrics

A significant illness — particularly one that lasts more than a few days or produces very high glucose elevations — will affect the next A1C result. The A1C test reflects average blood glucose over approximately three months, so a single week of glucose 100–150 mg/dL above the usual level has a more modest effect on the three-month average than a prolonged period of worsened control. However, a severe illness with glucose persistently above 300 mg/dL for a week or more can meaningfully elevate the next A1C result — by half a percentage point or more — even if control before and after the illness was excellent. When reviewing an A1C result that appears worse than expected, asking “Was there a significant illness in the past three months?” is an important clinical question that can explain an otherwise puzzling deterioration in the glucose average. Our guide on the A1C test explains how this three-month averaging works and how to interpret changes in A1C in the context of recent events. The connection between illness, temporary glucose elevation, and long-term health risk is addressed in our guide on why blood sugar matters for long-term health, which provides context for understanding why even temporary significant hyperglycemia during illness contributes to cumulative vascular risk — reinforcing the value of active sick-day management rather than passive tolerance of illness-related glucose elevation. For the full picture of blood sugar management that integrates sick-day protocols with day-to-day monitoring, our guide on home blood sugar monitoring and our guide on the blood sugar chart for adults provide the reference framework and practical tools that make active management during illness both feasible and effective.

Returning to Normal After Illness: What to Expect

Once an illness resolves and normal eating and activity are restored, blood glucose typically returns to its pre-illness baseline within 24–72 hours in most people with well-controlled diabetes. The counter-regulatory hormone surge that drove glucose elevation during the illness dissipates as the body recovers from the physiological stress of infection or inflammation, and insulin sensitivity — which was temporarily reduced by elevated cortisol, growth hormone, and glucagon during illness — gradually normalizes. People who were running higher correction insulin doses or monitoring more frequently during illness should step down both monitoring intensity and insulin dosing as glucose readings return to the pre-illness target range, rather than maintaining sick-day protocols once the illness is clearly resolved. One practical consideration during recovery: resuming held medications (particularly metformin or SGLT-2 inhibitors) should be confirmed with the prescribing provider, who may want to verify kidney function before restarting metformin and ensure adequate hydration before restarting SGLT-2 inhibitors. If glucose does not return to the pre-illness level within a few days of apparent clinical recovery — or if glucose is persistently higher than before the illness despite feeling well — this is worth discussing with a healthcare provider, as it may indicate that the illness has revealed or accelerated a worsening of underlying glucose control that warrants a medication review. Our guide on dry mouth and high blood sugar provides additional context on how the persistent dehydration that often lingers during recovery from gastrointestinal illness continues to affect glucose readings and what to do about it. Tracking glucose patterns during the recovery phase alongside notes about food intake and activity resumes the kind of systematic self-monitoring that makes long-term diabetes management effective — and the illness episode itself, while disruptive, often reinforces the importance of that monitoring as people see firsthand how dramatically non-dietary factors affect blood sugar.

Sources: American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1):S20–S42. • Kitabchi AE, et al. Hyperglycemic Crises in Adult Patients with Diabetes. Diabetes Care. 2009;32(7):1335–1343. • Muller LMAJ, et al. Increased Risk of Common Infections in Patients with Type 1 and Type 2 Diabetes. Clin Infect Dis. 2005;41(3):281–288.