What Raises Blood Sugar: Causes and UK Advice

What raises blood sugar is a question relevant to millions of people in the UK, whether they are managing diabetes, living with prediabetes, or simply trying to protect their long-term health. Blood glucose rises in response to a wide range of factors, including certain foods and drinks, physical and psychological stress, illness, sedentary habits, prescribed medicines, and underlying medical conditions. Understanding these triggers helps people make informed choices and recognise when to seek medical advice. This article explains the main causes of raised blood sugar, the UK diagnostic thresholds that matter, and when to contact a GP or seek urgent care.

What raises blood sugar levels and why does it matter?

Blood sugar rises when the body absorbs more glucose than cells can use, driven by food, hormones, stress, illness, and medicines. Persistently elevated levels damage blood vessels and nerves, and an HbA1c of 48 mmol/mol or above indicates type 2 diabetes.

Blood sugar levels rise whenever the body absorbs more glucose than its cells can use or store at that moment, which happens in response to food, hormones, stress, illness, and certain medicines. Glucose enters the bloodstream after carbohydrates are digested, and the pancreas responds by releasing insulin to move that glucose into cells for energy. When this system is disrupted, whether by diet, lifestyle, or an underlying condition, blood glucose can climb higher than is healthy.

Maintaining blood sugar within a normal range matters because persistently elevated levels damage blood vessels and nerves over time. The NHS states that a normal fasting blood glucose is generally 4.0 to 5.9 mmol/L for most adults without diabetes. A single reading above this range is not diagnostic on its own; if readings are persistently elevated, a GP can arrange formal testing, typically an HbA1c blood test or a fasting plasma glucose measurement. In the UK, an HbA1c of 48 mmol/mol or above, or a fasting plasma glucose of 7.0 mmol/L or above, is used to diagnose type 2 diabetes. An HbA1c of 42 to 47 mmol/mol, or a fasting plasma glucose of 6.1 to 6.9 mmol/L, indicates non-diabetic hyperglycaemia (sometimes called prediabetes), a state in which blood glucose is above normal but below the diagnostic threshold for type 2 diabetes. These thresholds are set out in NICE guidance (NG28) and NHS diagnostic criteria.

Non-diabetic hyperglycaemia is common in the UK and often produces no obvious symptoms, which is why NHS Health Checks and routine blood tests are important for people with risk factors such as obesity, a family history of type 2 diabetes, or a history of gestational diabetes. Identifying and addressing the factors that push blood sugar up is an important part of both prevention and management.

Which foods and drinks cause the biggest spikes in blood sugar?

High-GI foods such as white bread, sugary drinks, and fruit juice cause the largest, fastest blood sugar spikes. Pairing carbohydrates with protein, fat, or fibre slows glucose absorption and blunts the response.

Foods and drinks that are high in rapidly digested carbohydrates cause the largest and fastest rises in blood sugar. These include white bread, white rice, sugary breakfast cereals, cakes, biscuits, sweets, and sugar-sweetened drinks such as fizzy drinks, fruit juices, and energy drinks. The glycaemic index (GI) is a useful guide: high-GI foods are broken down quickly and produce sharp glucose spikes, while low-GI foods release glucose more gradually. Diabetes UK and the NHS both provide patient-facing guidance on GI and food choices for blood sugar management.

Some specific examples worth noting:

• Sugary drinks are particularly problematic because liquid carbohydrates are absorbed very rapidly and bypass the satiety signals that solid food triggers.

• Refined starchy foods such as white pasta, instant mashed potato, and processed snacks have a high GI and offer little fibre to slow glucose absorption.

• Fruit juice, even unsweetened varieties, can raise blood sugar quickly because the natural sugars are concentrated and the fibre present in whole fruit has been removed.

• Alcohol has a complex effect: beer and sweet wines can raise blood sugar initially, while spirits and excessive alcohol intake can later cause hypoglycaemia, particularly in people taking insulin or sulfonylureas.^[4] The NHS provides specific alcohol guidance for people with diabetes.

Portion size matters as much as food type. A large serving of a moderate-GI food can raise blood sugar more than a small serving of a high-GI food. Pairing carbohydrates with protein, healthy fats, or fibre slows digestion and blunts the glucose response. NICE and Diabetes UK both recommend a diet rich in vegetables, wholegrains, legumes, and lean protein as part of blood sugar management.

Can stress, illness, and lifestyle habits push blood sugar up?

Yes. Stress and illness release cortisol and adrenaline, which prompt the liver to release glucose into the bloodstream. Physical inactivity, poor sleep, smoking, and dehydration also raise blood sugar independently of diet.

Stress, illness, and several everyday lifestyle habits can all raise blood sugar, even in people who eat carefully. When the body perceives physical or psychological stress, it releases hormones including cortisol and adrenaline. These hormones trigger the liver to release stored glucose into the bloodstream as part of the fight-or-flight response, raising blood sugar independently of food intake. Chronic psychological stress, such as that caused by work pressure, poor sleep, or anxiety, can therefore contribute to persistently elevated glucose levels.

Illness has a similar effect. Infections, surgery, and other physical stressors cause the body to release stress hormones, which can push blood sugar up significantly. People with diabetes are advised by the NHS to monitor their blood glucose more frequently when unwell. Importantly, sick-day rules differ by medicine: insulin should generally be continued even if eating less, but some medicines, including metformin and SGLT2 inhibitors, may need to be temporarily stopped during acute illness or dehydration because of the risk of serious complications.^[1][2] You should not stop or adjust any prescribed medicine without first seeking advice from your GP or diabetes care team, who can give guidance tailored to your situation. NHS and Diabetes UK sick-day rules provide further detail.

Lifestyle factors that raise blood sugar include:

• Physical inactivity: Muscles use glucose during exercise, so a sedentary lifestyle reduces the body's ability to clear glucose from the blood efficiently.

• Poor sleep: Short or disrupted sleep impairs insulin sensitivity, meaning the body needs more insulin to achieve the same glucose-lowering effect.

• Smoking: Evidence links smoking to insulin resistance and an increased risk of type 2 diabetes.^[18][19]

• Dehydration: When fluid intake is low, blood glucose can become more concentrated. Dehydration and raised blood sugar often occur together during illness, partly because counter-regulatory hormones released during illness also drive glucose higher.

Addressing these modifiable factors through regular physical activity, adequate sleep, and stress management techniques forms a core part of NICE guidance (PH38) on type 2 diabetes prevention.^[5][6]

How do medicines and medical conditions affect blood sugar levels?

Corticosteroids are among the most significant medicines that raise blood sugar by reducing insulin sensitivity. Conditions including PCOS, Cushing's syndrome, and pancreatitis also elevate glucose independently of diet or lifestyle.

Several commonly prescribed medicines can raise blood sugar as a recognised side effect, and this is an important consideration for anyone at risk of or living with diabetes. Corticosteroids such as prednisolone are among the most significant: they reduce insulin sensitivity and can cause steroid-induced hyperglycaemia even in people with no prior history of diabetes. The effect is usually dose-dependent and may resolve when the steroid is stopped, but some people require temporary or long-term diabetes treatment during a course of steroids. The Joint British Diabetes Societies (JBDS) have published specific guidance on managing hyperglycaemia associated with glucocorticoid therapy.

Other medicines associated with raised blood sugar include:

• Thiazide diuretics (used for blood pressure and fluid retention) at higher doses

• Beta-blockers, which can impair insulin secretion and may also mask some of the warning symptoms of hypoglycaemia, making both effects clinically relevant

• Antipsychotic medicines, particularly second-generation agents such as olanzapine and clozapine, which are associated with weight gain and insulin resistance^[8][9]

• Some immunosuppressants used after organ transplantation, such as tacrolimus

Statins are also associated with a small increase in blood glucose and HbA1c, though the cardiovascular benefit of statins generally outweighs this effect for most people.^[11][12] The BNF and NICE CKS provide further detail on medicines that can affect glycaemic control.

If you are prescribed a new medicine and notice changes in your blood sugar readings, discuss this with your GP rather than stopping the medicine without advice.

A number of medical conditions also raise blood sugar independently of diet or lifestyle. Polycystic ovary syndrome (PCOS) is strongly associated with insulin resistance. Cushing's syndrome, caused by excess cortisol, raises glucose directly. Pancreatitis and pancreatic cancer can impair insulin production. Acromegaly (excess growth hormone) and hyperthyroidism both increase glucose output from the liver. Identifying and treating the underlying condition is central to managing blood sugar in these cases, and NICE provides condition-specific guidance for each.

When should you be concerned about high blood sugar in the UK?

Contact your GP if you have persistent symptoms such as increased thirst, frequent urination, or fatigue, especially with known diabetes risk factors. Call 999 immediately if DKA or HHS is suspected.

You should contact your GP if you experience persistent symptoms that may suggest high blood sugar, particularly if you have known risk factors for diabetes such as obesity, a family history of type 2 diabetes, or a previous diagnosis of non-diabetic hyperglycaemia or gestational diabetes. Common symptoms of hyperglycaemia include increased thirst, frequent urination, unexplained fatigue, blurred vision, and slow wound healing. These symptoms do not always appear, especially in the early stages, which is why NHS Health Checks and routine blood tests are important for at-risk groups.

For people already diagnosed with diabetes, the following situations warrant prompt contact with a GP or diabetes care team:

• Blood glucose readings consistently above your agreed target range

• Readings above 15 mmol/L on more than one occasion without an obvious cause; at this level, people with type 1 diabetes (and anyone unwell with high readings) should also check for ketones, as raised ketones alongside high blood sugar can indicate the early stages of diabetic ketoacidosis (DKA)^[1]

• Symptoms of illness combined with high readings that are not coming down

• Signs of a urinary tract or other infection, which can destabilise blood sugar control

If you are pregnant and concerned about high blood sugar, contact your maternity or diabetes team promptly, as thresholds and actions differ in pregnancy.

Diabetic ketoacidosis (DKA) is a medical emergency that can develop rapidly, most commonly in people with type 1 diabetes but occasionally in those with type 2 diabetes.^[1] Symptoms include vomiting, abdominal pain, rapid breathing, and confusion. If DKA is suspected, call 999 immediately.

Hyperosmolar hyperglycaemic state (HHS) is a separate emergency that occurs mainly in people with type 2 diabetes. It develops more slowly than DKA but can cause profound thirst, confusion, very high blood glucose, and severe dehydration. HHS also requires emergency medical attention: call 999 if these features are present.

For urgent advice that does not require an emergency response, NHS 111 is available around the clock.

If you are concerned about your blood sugar and are exploring options for assessment or weight management support, Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online consultations with prescriber review, where clinically appropriate treatment options may be considered alongside lifestyle advice. Suspected side effects from any medicine affecting blood sugar can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. High blood sugar (hyperglycaemia).
2. Type 2 diabetes in adults: management (NG28).
3. Blood glucose management | Type 2 diabetes in adults (NG28).
4. Low blood sugar (hypoglycaemia).
5. Type 2 diabetes: prevention in people at high risk (PH38).
6. Type 2 diabetes: prevention in people at high risk – Recommendations (PH38).
7. Type 2 diabetes | Quality standards (QS209) – Preventing type 2 diabetes.
8. Psychoses and related disorders | Treatment summaries – BNF.
9. Psychosis and schizophrenia in adults (CG178) – 2023 exceptional surveillance.
10. Pharmacogenetics of antipsychotic-induced weight gain: review and clinical implications.
11. Cardiovascular disease: risk assessment and reduction (NG238) – Recommendations.
12. Statins: risk of hyperglycaemia and diabetes.
13. Effects of statin therapy on diagnoses of new-onset diabetes and worsening glycaemia in large-scale randomised blinded statin trials: an individual participant data meta-analysis.
14. Tacrolimus | Drugs – BNF.
15. Tacrolimus induces insulin receptor substrate 1 hyperphosphorylation and inhibits mTORc1/S6K1 cascade in HL7702 cells.
16. Effect of histone deacetylase inhibitor (vorinostat) on new-onset diabetes induced by tacrolimus.
17. Current Pharmacological Intervention and Medical Management for Diabetic Kidney Transplant Recipients.
18. Smokeless tobacco (snus) is associated with an increased risk of type 2 diabetes: results from five pooled cohorts.
19. Evaluation of the effects of vitamin D deficiency and cigarette smoking on insulin resistance in type 2 diabetes mellitus: A meta-analysis of randomized controlled trials.

Frequently Asked Questions