The Impact of Illness and Infections on Blood Sugar Levels and Hypoglycemia Risk

When a person with diabetes develops any type of infection or illness—from the common cold to a urinary tract infection or stomach bug—the body’s reaction can trigger significant changes in blood sugar. Many people focus on the risk of high blood sugar during sickness, but the danger of hypoglycemia (dangerously low blood sugar) is equally important. Understanding the mechanisms behind these fluctuations and knowing how to respond can mean the difference between a manageable sick day and a medical emergency. This article explores why illness causes blood sugar volatility, why hypoglycemia risk rises, and the concrete steps you can take to stay safe.

The Physiological Response to Illness in Diabetes

Illness sets off a cascade of physiological responses that directly influence glucose metabolism. These responses vary depending on the type and severity of the infection, the medications taken, and the individual’s baseline diabetes control. The immune system’s activation alters hormone levels, fluid balance, and energy utilization in ways that can destabilize glucose in both directions.

Stress Hormones and Insulin Resistance

When the immune system battles an infection, it releases stress hormones such as cortisol and adrenaline. These hormones signal the liver to release stored glucose into the bloodstream, providing energy for fighting the infection. At the same time, they reduce the sensitivity of cells to insulin—a state called insulin resistance. For people with diabetes who rely on exogenous insulin or oral medications that increase insulin secretion, this hormone surge can cause blood sugar to spike unexpectedly. Even mild infections like a sinus infection can push blood glucose levels above target range, while a serious illness such as pneumonia can lead to prolonged hyperglycemia if not managed carefully.

Inflammatory Cytokines and Glucose Metabolism

In addition to stress hormones, the immune system releases inflammatory cytokines (e.g., interleukin-6, tumor necrosis factor-alpha) that interfere with insulin signaling at the cellular level. This inflammatory response further amplifies insulin resistance and can also stimulate the liver to produce more glucose through gluconeogenesis. The combination of stress hormones and cytokines creates a powerful hyperglycemic drive that may persist for days, even after the fever breaks. People with poorly controlled diabetes already have chronic low-grade inflammation, making them especially vulnerable to exaggerated glucose rises during acute infections.

Dehydration and Electrolyte Imbalance

Fever, vomiting, diarrhea, and excessive sweating from illness all contribute to fluid loss. Dehydration concentrates the blood, which can elevate glucose readings temporarily. More important, dehydration impairs kidney function and reduces the body’s ability to clear excess glucose through urine. Simultaneously, electrolyte imbalances—especially low potassium or sodium—can affect how insulin works and how cells take up glucose. This creates a volatile situation where blood sugar can swing high and low within hours, making monitoring even more critical. Hypokalemia, in particular, can blunt the action of insulin and predispose to cardiac arrhythmias, adding another layer of risk during illness.

Reduced Appetite and Altered Food Intake

Many illnesses suppress appetite or cause nausea, leading to reduced food and carbohydrate intake. When a person skips meals or eats far less than usual, their expected insulin or medication dose may be too high relative to the actual glucose load. This mismatch is a primary cause of hypoglycemia during illness. Additionally, some infections change how quickly the stomach empties, further disrupting the timing between food, medication, and glucose absorption. It’s not uncommon for a person to experience both high fasting glucose (from stress hormones) and low postprandial glucose (from inadequate food) on the same day. Gastroparesis, a common complication of diabetes, can worsen these delays, making sick-day management even more challenging.

Hypoglycemia: A Hidden Danger During Sickness

While many people with diabetes worry about high blood sugar when they are ill, hypoglycemia poses an equal or greater acute risk. Severe hypoglycemia can lead to confusion, seizures, unconsciousness, and even death. During illness, the warning signs may be masked or misinterpreted, and the usual corrective strategies may not work as expected. The danger is compounded by the fact that patients, families, and even some clinicians focus primarily on hyperglycemia and ketoacidosis, overlooking the low blood sugar risk until a crisis occurs.

Why Hypoglycemia Risk Increases

Several factors converge during illness to raise the likelihood of low blood sugar:

  • Reduced caloric intake: Nausea, vomiting, or sore throat can decrease food consumption, leaving less glucose available to match medication doses.
  • Medication timing errors: People may take their usual insulin or oral agents despite not eating, or they may inadvertently double-dose because they forget whether they already injected.
  • Increased glucose clearance: Fever increases metabolic rate and may accelerate glucose utilization by immune cells, while some infections (like gastroenteritis) speed up intestinal transit, reducing glucose absorption.
  • Kidney function changes: Dehydration reduces renal clearance of insulin and some oral hypoglycemic drugs, prolonging their action and increasing hypoglycemia risk. This is especially important for medications like sulfonylureas and long-acting insulin analogs.
  • Alcohol or drug interactions: Some people use alcohol-containing cough syrups or cold medicines that can either raise or lower blood sugar, adding another variable. Even sugar-free medicines may contain caffeine or decongestants that affect glucose.
  • Insulin sensitivity from rest: Bed rest may increase insulin sensitivity in some individuals, leading to lower glucose levels if medication doses are not adjusted.
  • Delayed stomach emptying: Medications like GLP-1 receptor agonists slow gastric emptying normally; when combined with illness-related gastroparesis, the mismatch between insulin and glucose absorption can become extreme.

Recognizing Hypoglycemia When Sick

During illness, the classic hypoglycemia symptoms—shakiness, sweating, rapid heartbeat, hunger, and anxiety—may be attributed to the fever or infection itself. This makes it essential to rely on blood glucose monitoring rather than symptoms alone. Any reading below 70 mg/dL (3.9 mmol/L) requires immediate intervention. Be especially vigilant if you experience confusion, slurred speech, unusual behavior, or if you cannot keep food or fluids down—these signal a need for urgent care. In older adults or those with long-standing diabetes, hypoglycemia may present atypically as dizziness, weakness, or falls, mimicking the effects of the illness itself. Use a CGM with low-glucose alerts if available, but confirm with a fingerstick before treating, as CGM lag can be significant during illness when circulation and interstitial fluid dynamics are altered.

Illness Management by Diabetes Type

The approach to sick-day management differs markedly between type 1 diabetes, type 2 diabetes, and other forms like gestational diabetes. Understanding these distinctions helps tailor the response.

Type 1 Diabetes

People with type 1 diabetes produce no endogenous insulin and are entirely dependent on exogenous insulin. During illness, the risk of diabetic ketoacidosis (DKA) is extremely high because even a few hours without sufficient insulin can lead to ketone production. At the same time, reduced food intake can cause profound hypoglycemia if insulin doses are not adjusted. The key is to never omit basal insulin entirely; instead, adjust bolus doses based on frequent glucose and ketone monitoring. Many guidelines recommend using correction doses of rapid-acting insulin every 2–4 hours, guided by ketone levels. For persistent vomiting or high ketones, emergency department evaluation is mandatory.

Type 2 Diabetes

In type 2 diabetes, the pancreas still produces some insulin, so the risk of DKA is lower but not zero—especially in those using SGLT2 inhibitors, who can develop euglycemic DKA. The primary dangers during illness are hyperosmolar hyperglycemic state (HHS) and hypoglycemia from medication mismatch. Metformin is generally safe to continue, but it may be held if there is significant vomiting or dehydration to avoid lactic acidosis. Sulfonylureas and meglitinides carry the highest hypoglycemia risk and may need dose reduction if caloric intake drops. Insulin users should follow similar principles as type 1, though the doses are often lower relative to body weight. For mild illness, many patients with well-controlled type 2 can temporarily increase their basal insulin by 10–20% to counter stress-induced hyperglycemia.

Gestational Diabetes

Pregnant women with gestational diabetes face unique challenges because illness can affect both maternal glucose and fetal well-being. The same stress hormone surge occurs, but insulin resistance is already high in pregnancy. Hypoglycemia is less common than in other diabetes types because of the intrinsic insulin resistance of pregnancy, but it can still occur if food intake is poor or if the woman is using insulin or glyburide. Ketone monitoring is critical because maternal ketones can cross the placenta and affect fetal development. Pregnant women should contact their obstetrician or endocrinologist early in the illness to adjust their plan and should seek care if glucose remains above 180 mg/dL or if ketones appear.

Medication Adjustments During Illness

Never stop taking your diabetes medications without consulting a healthcare professional. However, dosages often need to be modified. The following table summarizes general guidance for common drug classes during illness. Always individualize based on glucose trends, ketones, and clinical status.

  • Insulin (all types): Continue basal insulin at or near usual dose to prevent ketoacidosis. Rapid-acting bolus doses may need to be increased if hyperglycemia dominates, or decreased if food intake is severely reduced. Many providers recommend a sick-day sliding scale with insulin doses based on glucose and ketones every 2–4 hours.
  • Metformin: Generally safe to continue unless vomiting, severe diarrhea, or dehydration increase the risk of lactic acidosis. Check with your doctor if you have kidney impairment or are very ill. If holding metformin, monitor glucose and ketones closely.
  • Sulfonylureas (e.g., glipizide, glyburide, glimepiride): These drugs can cause hypoglycemia, especially if food intake is low. Doses may need to be reduced by 50% or temporarily held until caloric intake normalizes. Use caution in elderly patients and those with renal impairment.
  • Meglitinides (repaglinide, nateglinide): Similar to sulfonylureas, these should be held if meals are skipped. Take only if you are eating or plan to eat within 30 minutes.
  • SGLT2 inhibitors (e.g., empagliflozin, dapagliflozin, canagliflozin): These can increase the risk of euglycemic DKA (normal blood sugar but with ketones) during illness. Many experts advise stopping them 24–48 hours before planned illness or immediately at onset of acute illness, especially if nausea, vomiting, or reduced oral intake occurs. Restart only after the individual is eating and drinking normally and ketones are negative.
  • GLP-1 receptor agonists (e.g., semaglutide, liraglutide, dulaglutide): They have a low intrinsic hypoglycemia risk but can slow gastric emptying and worsen nausea. Adjust doses only on medical advice; some clinicians hold the dose until appetite returns.
  • DPP-4 inhibitors (sitagliptin, linagliptin, etc.): Generally low risk for hypoglycemia and safe to continue during mild illness. No dose adjustment is usually needed, but monitor glucose.
  • Thiazolidinediones (pioglitazone): Rarely cause hypoglycemia alone, but fluid retention can worsen in the setting of illness and heart failure. Continue with caution, especially if there is edema or dyspnea.

Always document what and when you took, and keep a record to share with your healthcare team. If you use an insulin pump, consider having a backup plan for injections in case the pump site fails or the pump malfunctions during illness.

The Role of Continuous Glucose Monitoring During Illness

Continuous glucose monitoring (CGM) provides invaluable real-time data during sickness, but it has limitations that users must understand. Illness can alter the relationship between interstitial glucose and blood glucose due to changes in perfusion, hydration, and pH. Dehydration may cause the CGM sensor to read lower than actual blood glucose, while fever can accelerate glucose kinetics and cause lag times of up to 20 minutes. Always confirm CGM readings with a fingerstick before making treatment decisions, especially for hypoglycemia or hyperglycemia with ketones. Set your CGM alerts to a wider range (e.g., high alert at 200 mg/dL, low alert at 80 mg/dL) to give yourself more time to respond. If your CGM shows rapid drops of more than 2 mg/dL per minute, treat for impending hypoglycemia even if the absolute number is above 70 mg/dL.

Special Considerations: Children, Elderly, and Pregnancy

Vulnerable populations require extra attention during illness because they have less physiologic reserve and may be less able to communicate symptoms.

Children with Diabetes

Children are more likely to get common infections and have smaller glycogen stores, making hypoglycemia a particular concern. They may not recognize or articulate low blood sugar symptoms. Parents should check glucose every 1–2 hours when the child is sick, and ensure the school or daycare has a written sick-day plan. Glucagon mini-doses (0.5 mg for children under 25 kg) can be used for severe hypoglycemia that does not respond to oral glucose. For hyperglycemia with ketones, follow a pediatric sick-day protocol from a pediatric endocrinologist. The risk of DKA in children with type 1 diabetes is especially high because they may develop ketones within hours of insufficient insulin.

Elderly Adults with Diabetes

Older adults often have multiple comorbidities, reduced kidney function, and are on multiple medications that can interact during illness. Hypoglycemia in the elderly can present as confusion, falls, or stroke-like symptoms, and can be mistaken for dementia or delirium. They are also at higher risk for medication errors, especially if they live alone or have cognitive impairment. Caregivers should supervise medication administration during illness. The use of sulfonylureas and long-acting insulin should be carefully reviewed, and lower glucose targets (100–180 mg/dL) may be acceptable to avoid hypoglycemia during acute illness.

Pregnancy

As noted, pregnant women with any form of diabetes should have a low threshold for seeking medical attention during illness. Dehydration can lead to preterm labor, and hyperglycemia increases the risk of congenital anomalies and macrosomia. Check ketones at least twice daily and aim for glucose levels between 70 and 140 mg/dL if safe. Insulin adjustments should be made in consultation with the obstetric diabetes team. Avoid SGLT2 inhibitors and GLP-1 agonists in pregnancy altogether, per current guidelines.

Preventing Infections to Stabilize Blood Sugar

While you cannot avoid all illnesses, reducing your risk of infection helps maintain better long-term diabetes control. Preventive measures include:

  • Staying up to date on vaccinations: influenza, pneumococcal pneumonia, COVID-19, hepatitis B, and tetanus among others. The CDC recommends that all adults with diabetes receive the pneumococcal conjugate vaccine and an annual influenza shot.
  • Practicing good hand hygiene—wash with soap and water or use alcohol-based sanitizer frequently
  • Avoiding close contact with people who are sick
  • Keeping blood glucose well-controlled before an infection occurs, as poor glycemic control impairs immune function
  • Regular dental checkups to prevent gum infections, which can raise blood sugar
  • Inspecting feet daily and promptly treating any cuts, blisters, or sores
  • Managing stress and sleep, as these also affect immune function

When to Seek Medical Attention

Recognizing when home management is no longer sufficient can save a life. Contact your healthcare provider or go to the emergency department if:

  • Blood glucose remains above 300 mg/dL (16.7 mmol/L) despite correcting hydration and taking medications as advised
  • You have moderate to large urine ketones or blood ketones above 0.6 mmol/L
  • You are vomiting or have diarrhea for more than 6 hours and cannot keep fluids down
  • You have a fever above 101°F (38.3°C) that does not respond to over-the-counter medications
  • You experience persistent confusion, difficulty breathing, or chest pain
  • You have had several episodes of severe hypoglycemia (requiring assistance) or have not been able to keep glucose above 70 mg/dL despite eating
  • You are unsure how to adjust your medications or your condition is worsening
  • You are pregnant and have ketones or glucose >180 mg/dL for more than 2 hours

Conclusion

Illness and infections place extraordinary stress on the body, creating unpredictable swings in blood sugar that increase the risk of both hyperglycemia and hypoglycemia. The interplay between stress hormones, dehydration, reduced food intake, and medication dynamics demands continuous vigilance. By preparing a personalized sick day plan, monitoring glucose and ketones frequently, adjusting medications under medical guidance, and knowing when to seek help, people with diabetes can navigate illness more safely. The key is to act early, stay hydrated, and communicate clearly with your healthcare team. With the right knowledge and tools, you can minimize the impact of illness on your diabetes and prevent dangerous complications.

For further guidance, consult resources such as the American Diabetes Association Sick Day Rules and the CDC Diabetes and Sick Days page. For detailed medication management, see Diabetes UK sick day guidance and the NIDDK sick day management overview. Additional specialized guidance is available from the JDRF Sick Day Management Toolkit for Type 1 Diabetes and the Endocrine Society guidelines on inpatient diabetes management, which contain principles applicable to sick days at home.