Why do hospitals give insulin instead of metformin?

October 11, 2025 •

4 min read

According to the American Diabetes Association, nearly one in four hospitalized patients has diabetes. In contrast to outpatient management, hospitals predominantly switch from oral medications like metformin to insulin to manage blood glucose, a strategy based on both patient safety and the physiological demands of acute illness.

Quick Summary

Hospitals switch patients from metformin to insulin primarily due to safety concerns and insulin's effectiveness for rapidly changing health needs. Metformin is held to prevent rare but serious lactic acidosis, especially with kidney impairment or poor tissue oxygenation. Insulin provides flexible, precise, and immediate glucose control for critically or acutely ill patients.

Key Points

Lactic Acidosis Risk: Hospital conditions like kidney dysfunction, sepsis, and heart failure significantly increase the risk of metformin-associated lactic acidosis, a rare but severe complication.
Precise Control with Insulin: Intravenous insulin infusions allow for immediate and precise dose adjustments, which is essential for managing unpredictable and rapid changes in blood glucose during acute illness.
Management of Stress Hyperglycemia: The body's stress response to illness or surgery can cause temporary insulin resistance and elevated blood sugar (stress hyperglycemia), which insulin is uniquely capable of overcoming.
Flexibility with Nutritional Status: Insulin is effective regardless of whether a patient is eating, fasting, or receiving IV fluids, offering flexibility that metformin lacks.
Transition at Discharge: Patients can typically be transitioned back to their oral metformin regimen as they recover and stabilize, provided the risk factors for lactic acidosis have resolved.
Standard Hospital Protocol: Using insulin is the standard of care for inpatient diabetes management, prioritizing safety and tight glycemic control during a vulnerable period for the patient.

The Foundation of Outpatient vs. Inpatient Care

In outpatient care, metformin is the standard first-line oral medication for type 2 diabetes due to its effectiveness in lowering blood glucose levels, favorable cardiovascular benefits, and low risk of hypoglycemia. It works primarily by decreasing glucose production in the liver and improving insulin sensitivity. This slow, steady action is well-suited for long-term, stable management. However, the dynamics change completely in a hospital environment where a patient’s health status can be rapidly altered by acute illness, surgery, or other medical stressors.

Metformin's Inpatient Risks and Limitations

For hospitalized patients, several conditions can increase the risk of a severe side effect known as metformin-associated lactic acidosis (MALA), a life-threatening condition caused by a buildup of lactic acid in the body. This risk is the single most important reason clinicians hold metformin upon hospital admission. Key risk factors for MALA that are common among hospitalized patients include:

Kidney Impairment: Metformin is eliminated by the kidneys. Any condition causing acute kidney injury (AKI), such as dehydration, sepsis, or a severe infection, can lead to metformin accumulation and increase the risk of MALA.
Tissue Hypoxia: Critically ill patients, especially those with severe infections, congestive heart failure, or respiratory failure, often experience poor oxygen delivery to tissues. This anaerobic state increases lactate production, raising the risk of lactic acidosis.
Iodinated Contrast Studies: Many medical procedures, including CT scans and angiograms, require the injection of a contrast dye. This dye can be toxic to the kidneys, prompting guidelines to recommend temporarily stopping metformin to prevent accumulation.
Liver Disease: Significant liver impairment can reduce the liver's ability to clear lactate from the blood, further increasing the risk of lactic acidosis in patients taking metformin.
Severe Illness (Sepsis): The systemic inflammatory response associated with sepsis can disrupt multiple organs, including the kidneys and liver, creating a high-risk environment for MALA.

Given the unpredictability of a hospitalized patient’s clinical course, discontinuing metformin is a standard safety measure to avoid these risks, even if the patient initially appears stable.

The Advantages of Insulin in the Hospital

In contrast to metformin, insulin is the preferred medication for glycemic control in both critically and non-critically ill hospitalized patients. Its advantages are a direct response to the challenges of inpatient care:

Rapid and Flexible Action: Insulin can be administered intravenously via a continuous infusion, allowing for rapid and precise dose adjustments based on real-time blood glucose monitoring. This is crucial for unstable patients or those who cannot eat. In less critical patients, subcutaneous injections can be tailored to their nutritional status.
Control Over Stress-Induced Hyperglycemia: Acute illness, trauma, and surgery trigger a stress response that releases hormones like cortisol and epinephrine. This can lead to temporary but significant insulin resistance and elevated blood glucose, known as stress hyperglycemia. Insulin therapy effectively counteracts this physiological response.
Predictable and Controllable Effects: Unlike metformin, insulin does not rely on organ function (like the kidneys) in the same way, making it a safer and more predictable option for patients with fluctuating kidney or liver function. It can be easily stopped if hypoglycemia occurs.
Nutrition Management: Many hospitalized patients have variable or nonexistent oral intake. With insulin, clinicians can adjust doses to cover glucose from intravenous fluids, tube feeds, or parenteral nutrition, providing consistent control regardless of diet.

Comparing Metformin and Insulin in an Inpatient Setting


Feature	Metformin (Oral)	Insulin (Injectable/IV)
Mechanism of Action	Decreases liver glucose production and increases insulin sensitivity.	Directly introduces insulin to promote glucose uptake by cells.
Speed of Effect	Slow, for long-term stability. Not for acute control.	Rapid, especially via IV, allowing for immediate titration.
Dose Adjustability	Fixed or slowly titrated dose. Not flexible for acute changes.	Highly flexible and can be rapidly adjusted for minute-to-minute needs.
Use in Kidney Failure	Contraindicated in severe renal impairment (eGFR <30 mL/min/1.73 m$^2$) due to lactic acidosis risk.	Safely used, with dosage adjusted for any changes in renal function.
Use in Acute Illness/Sepsis	High risk of lactic acidosis due to tissue hypoxia and organ dysfunction.	Preferred and effective therapy for managing stress hyperglycemia.
Nutritional Flexibility	Depends on consistent oral intake. Not suitable for NPO patients.	Allows for blood glucose management independent of nutritional intake.

Transitioning from Insulin Back to Metformin

As a patient's acute illness resolves and their nutritional intake and organ function stabilize, a hospital team will often transition them back to their home oral diabetes regimen, including metformin, before discharge. This process involves a careful assessment of the patient’s overall health and is a crucial step to ensure a smooth and safe transition back to outpatient care. The resumption of metformin typically occurs once the patient no longer has risk factors for lactic acidosis, such as severe renal dysfunction or hypoperfusion.

Conclusion: A Tailored Approach for Patient Safety

The choice to use insulin over metformin in the hospital is not a judgment on the effectiveness of metformin but a critical patient safety measure based on pharmacological principles and physiological responses to stress. Insulin's rapid action, superior flexibility, and lower risk profile in the setting of acute illness and organ instability make it the ideal tool for managing inpatient hyperglycemia. This tailored approach ensures that each patient receives the safest and most effective diabetes care for their specific clinical circumstances while hospitalized. For more information, the National Center for Biotechnology Information (NCBI) has resources on inpatient diabetes management.

Frequently Asked Questions

Yes, for many hospitalized patients, it is dangerous to continue metformin. The drug's risks, specifically metformin-associated lactic acidosis, are heightened by conditions common in the hospital, such as kidney impairment, severe infection, and poor oxygen delivery to tissues. For this reason, it is almost always discontinued upon admission.

No, metformin is an excellent and safe medication for long-term diabetes management in the outpatient setting, with benefits including cardiovascular risk reduction. The switch to insulin is a temporary, safety-driven measure based on the patient's acute medical needs during hospitalization.

Insulin doses in the hospital are highly individualized. For critically ill patients, continuous intravenous insulin infusions allow for constant, real-time adjustments based on frequent blood glucose monitoring. For stable patients, a basal-bolus regimen is often used, providing a daily background dose and mealtime doses, with supplemental insulin for corrections.

Stress hyperglycemia is a temporary rise in blood sugar that occurs in response to physical stress from illness, injury, or surgery. It is caused by the release of stress hormones like cortisol and adrenaline, which reduce the body's sensitivity to insulin. Insulin therapy is effective in treating this condition.

No, the switch is usually temporary. As the patient recovers from their acute illness and their organ function stabilizes, the healthcare team will likely transition them back to their home oral diabetes regimen, including metformin, before they are discharged.

While close monitoring is always performed, the risk of metformin-associated lactic acidosis in patients with specific comorbidities is considered too high to justify continued use. Given insulin's superior safety profile and effectiveness in acute care settings, it is the standard and safer choice.

Hypoglycemia is a potential risk with any insulin therapy, which is why blood sugar levels are monitored closely and frequently in the hospital. While intensive insulin therapy can increase this risk, modern protocols and careful titration are designed to keep blood glucose within a safe, controlled range of 140-180 mg/dL for most ICU patients.