Does Insulin Increase Blood Glucose?

Insulin does not increase blood glucose; it lowers it. This is one of the most important principles in diabetes management, yet confusion around insulin's role is common, particularly when blood glucose rises unexpectedly after an injection. Understanding how insulin works, why glucose levels can still climb despite treatment, and how NHS clinicians approach dose adjustment can help people with diabetes make sense of their readings and have more informed conversations with their care team. This article explains the physiology, the common causes of apparent treatment failure, and the clinical approach to keeping blood glucose stable.

What does insulin actually do to your blood glucose levels?

Insulin lowers blood glucose by unlocking cells so glucose moves from the bloodstream into muscle, fat and liver tissue, and by suppressing hepatic glucose production through gluconeogenesis and glycogenolysis.

Insulin lowers blood glucose, not raises it. Produced by the beta cells of the pancreas, insulin acts as a key that unlocks cells throughout the body, allowing glucose to move from the bloodstream into muscle, fat and liver cells where it is used for energy or stored as glycogen. Without sufficient insulin, glucose accumulates in the blood, which is the defining feature of diabetes mellitus.

The mechanism works through insulin receptors on cell surfaces. When insulin binds to these receptors, it triggers a cascade of intracellular signals. In muscle and adipose tissue, this causes glucose transporter proteins (particularly GLUT4) to migrate to the cell membrane, enabling glucose uptake. In the liver, insulin acts differently: it suppresses the production of new glucose (gluconeogenesis), promotes glycogen synthesis, and reduces the breakdown of glycogen back into glucose (glycogenolysis), primarily through effects on hepatic enzymes and signalling pathways rather than via GLUT4.^[1][4]

In people without diabetes, the pancreas releases insulin in a tightly regulated pattern: a low background (basal) level throughout the day, with larger pulses after meals to handle the glucose absorbed from food. Modern insulin therapy attempts to replicate this pattern using combinations of long-acting basal insulins and rapid-acting bolus insulins. Understanding that insulin's primary role is glucose reduction is essential for anyone managing diabetes, as it underpins every decision about dosing, timing and lifestyle.

Why does blood glucose sometimes rise after an insulin injection?

Blood glucose can rise after an insulin injection due to an insufficient dose, lipohypertrophy impairing absorption, illness, stress, or counter-regulatory hormone surges such as the dawn phenomenon or Somogyi effect.

Blood glucose can rise after an insulin injection for several well-recognised reasons, even though insulin itself acts to lower glucose. The most common explanation is that the dose was insufficient for the amount of carbohydrate consumed, the level of physical inactivity, or the degree of insulin resistance present at that time.

One physiological cause sometimes cited is the Somogyi effect (rebound hyperglycaemia), where an episode of low blood glucose (hypoglycaemia) triggers a counter-regulatory hormone response. The body releases glucagon, adrenaline, cortisol and growth hormone, all of which stimulate the liver to release stored glucose.^[6] However, the evidence for true rebound hyperglycaemia after nocturnal hypoglycaemia is debated, and this pattern is considered less common than the dawn phenomenon.^[6][8] If you suspect overnight hypoglycaemia is causing a morning rise, checking a blood glucose reading at around 03:00, or using continuous glucose monitoring (CGM), can help clarify the cause before any insulin adjustment is made.

A separate phenomenon, the dawn phenomenon, causes blood glucose to rise in the early hours of the morning due to natural surges in growth hormone and cortisol, independent of any hypoglycaemic episode.^[7][8] Both patterns can mislead people into thinking their insulin is not working or is causing the rise.

Other factors that can blunt insulin's effect and lead to apparent post-injection hyperglycaemia include:

• Injection into lipohypertrophic (scarred) tissue, which impairs absorption^[10]

• Incorrect storage of insulin (follow the storage instructions in your insulin's patient information leaflet, as requirements vary between products)

• Illness or infection, which raises counter-regulatory hormones

• Psychological stress, which elevates cortisol

• Certain medicines, including corticosteroids and some antipsychotics, which are recognised to raise blood glucose

If blood glucose consistently rises after injections, this warrants a structured review with a diabetes specialist nurse or GP rather than self-adjustment of doses without guidance.

What are the signs that your insulin dose is not working properly?

Persistently elevated fasting or post-meal glucose, unexplained thirst, fatigue, recurrent hypoglycaemia, or a raised HbA1c despite adherence all suggest the insulin dose may need clinical review.

Persistently elevated blood glucose readings, particularly above the target range agreed with your diabetes care team, are the clearest sign that an insulin dose may not be working as intended. NICE guidance (NG17 for type 1 diabetes in adults and NG28 for type 2 diabetes in adults) recommends that people using insulin monitor their blood glucose regularly and keep a record to identify patterns over time.^[12][14]

Signs that may indicate an insulin dose problem include:

• Fasting blood glucose consistently above your individually agreed target (targets vary between people and should be set with your diabetes care team)

• Blood glucose remaining elevated two hours after meals, above the level agreed with your care team

• Unexplained fatigue, increased thirst or frequent urination, which are classic symptoms of hyperglycaemia

• Recurrent hypoglycaemic episodes, which may suggest the dose is too high rather than too low

• HbA1c results that remain above your agreed target despite apparent adherence to treatment

Physical signs at injection sites are also important. Lumpy, hardened areas of skin (lipohypertrophy) caused by repeated injections into the same spot can significantly reduce insulin absorption. Rotating injection sites systematically, as recommended in TREND-UK injection technique guidance and by Diabetes UK, helps prevent this.

If you notice any of the above, contact your GP or diabetes nurse for a review rather than adjusting your dose independently. If you experience symptoms of diabetic ketoacidosis (DKA), such as vomiting, rapid breathing, confusion or blood ketones above 3 mmol/L, call 999 or go to A&E immediately, as this is a medical emergency.

How do NHS clinicians adjust insulin to keep blood glucose stable?

NHS clinicians use an iterative, NICE-guided approach reviewing glucose patterns, HbA1c, injection technique and lifestyle before adjusting basal or bolus doses, with CGM recommended for most adults with type 1 diabetes.

NHS clinicians adjust insulin doses using a structured, evidence-based approach guided by NICE recommendations and the individual's blood glucose monitoring data, HbA1c results and lifestyle factors. The process is rarely a single adjustment; it is an iterative review that considers both fasting and post-meal glucose patterns, frequency of hypoglycaemia and the person's own preferences and capabilities.

For people with type 1 diabetes, NICE (NG17) recommends a basal-bolus regimen as the standard approach, with doses adjusted using carbohydrate counting and insulin-to-carbohydrate ratios.^[13][17] Continuous glucose monitoring (CGM) is now recommended by NICE (NG17) for most adults with type 1 diabetes, providing real-time data that makes dose adjustment more precise and reduces the risk of undetected hypoglycaemia.

For type 2 diabetes managed with insulin, NICE (NG28) advises starting with a once-daily basal insulin and titrating the dose incrementally, guided by fasting glucose readings, with the rate and size of adjustments determined by the individual's care plan.^[15][16] If fasting glucose is controlled but post-meal readings remain high, a rapid-acting insulin before meals may be added.

Key principles that clinicians apply include:

• Reviewing injection technique and site rotation before assuming a dose change is needed

• Identifying and addressing reversible causes of poor control (illness, new medicines, dietary changes)

• Setting individualised glucose targets, particularly for older adults or those with hypoglycaemia unawareness

• Involving the person in shared decision-making about any regimen change

Online consultations can support access to diabetes reviews. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online consultations with prescriber review and pharmacist support, where clinically appropriate care pathways can be discussed. Any suspected adverse effects of insulin should be reported via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. Insulin | Treatment summaries - BNF - NICE.
2. Divergent AKT Signalling Mechanisms Regulate GLUT4 Translocation and Glucose Uptake in Skeletal Muscle and Adipose Tissue.
3. Canonical and Alternative Pathways (Insulin and Exercise) of GLUT4 Synthesis, Signaling, Intracellular Clustering, and Recruitment to the Plasma Membrane.
4. Physiological and pathophysiological actions of insulin in the liver.
5. Insulin-Related Liver Pathways and the Therapeutic Effects of Aerobic Training, Green Coffee, and Chlorogenic Acid Supplementation in Prediabetic Mice.
6. Do high fasting glucose levels suggest nocturnal hypoglycaemia? (Somogyi effect).
7. Nocturnal spikes of growth hormone secretion cause the dawn phenomenon in type 1 (insulin-dependent) diabetes mellitus.
8. The dawn phenomenon in diabetes: pathophysiology from periphery to central nervous system and circadian rhythm-based therapeutic strategies.
9. Nocturnal blood glucose control in type I diabetes mellitus.
10. Diabetic lipohypertrophy delays insulin absorption.
11. Abnormalities in Glucose Regulation During Antipsychotic Treatment.
12. Type 1 diabetes in adults: diagnosis and management (NG17).
13. Type 1 diabetes in adults: diagnosis and management (NG17) – Recommendations.
14. Type 1 diabetes in adults: diagnosis and management (NG17) – Using insulin.
15. Type 2 diabetes in adults: management (NG28).
16. Insulin-based treatments | Type 2 diabetes in adults (NG28).
17. Type 1 diabetes | Treatment summaries - BNF - NICE.
18. Continuous glucose monitoring | Type 1 diabetes in adults (QS208).
19. Resource impact report for continuous glucose monitoring – type 1 and type 2 diabetes (NG17/NG18).
20. Diabetic hyperglycaemic emergencies | Treatment summaries - BNF - NICE.
21. Diabetic ketoacidosis – NHS.

Frequently Asked Questions