What Happens When Blood Sugar Stays High for Too Long
What Happens When Blood Sugar Stays High for Too Long
Short spikes are normal. Months and years above target are not. Here is what sustained high blood sugar does to your heart, kidneys, eyes, nerves and brain, which warning signs matter, when it becomes an emergency, and exactly what to do next.
An occasional high reading after a large meal, an infection, or a stressful week is common and usually does no lasting harm. The real concern is blood sugar that stays above target for months or years. Over time, high blood glucose damages blood vessels and the nerves that control your heart, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), and the same process reaches the kidneys, eyes and feet.
The difficult part is how quiet it is. Mayo Clinic notes that hyperglycemia usually does not cause symptoms until glucose is above about 180 to 200 mg/dL (10 to 11.1 mmol/L), and some people who have had type 2 diabetes for a long time feel nothing even when readings are high. The International Diabetes Federation estimates that about 252 million adults worldwide have diabetes without knowing it, so many people carry the damage for years before anyone looks for it.
There is no single number of days after which damage becomes certain. Risk is cumulative: it rises with how high your glucose runs and how long it stays there. That is also why this is good news in disguise. Every month you bring your average down is a month of protection.
This guide answers the questions people actually search: how high is too high, how long is too long, which organs are at risk, what the warning signs are, when to go to emergency care, what to do today, which tests to ask for, and what can realistically improve. It builds on our guides to what causes high blood sugar and how to reverse insulin resistance naturally. All glucose numbers are shown in both mg/dL and mmol/L.
Also read: What Is Type 2 Diabetes?
Urgent If Your Reading Is High Right Now
- Vomiting or diarrhea and you cannot keep food or fluids down
- Belly pain, fruity-smelling breath, fast or difficult breathing
- Confusion, extreme drowsiness, or fainting
- Glucose that stays above 240 mg/dL (13.3 mmol/L) with ketones in your urine or blood
These match Mayo Clinic's guidance on when to seek immediate medical care for hyperglycemia and the ADA's advice to seek urgent help when you cannot tolerate fluids or mental status changes. If none of these apply, go to Step 3 below for what to do safely today.
Data Key Numbers at a Glance
Numbers How High Is Too High?
“High” depends on whether you are being diagnosed or already managing diabetes. The first table shows the diagnostic ranges used by the American Diabetes Association and the CDC. The second shows what to do at different levels.
| Category | Fasting glucose | 2 hours after a glucose drink | A1C |
|---|---|---|---|
| Normal | Below 100 mg/dL (5.6 mmol/L) | Below 140 mg/dL (7.8) | Below 5.7% |
| Prediabetes | 100 to 125 mg/dL (5.6 to 6.9) | 140 to 199 mg/dL (7.8 to 11.0) | 5.7% to 6.4% |
| Diabetes | 126 mg/dL or higher (7.0) | 200 mg/dL or higher (11.1) | 6.5% or higher |
Ranges from the CDC's diabetes tests page (based on ADA criteria). A diagnosis is made by a clinician using laboratory tests and usually needs confirmation with a repeat test. A single home meter reading does not diagnose diabetes.
If you already have diabetes, the ADA Standards of Care 2026 give these general goals for many non-pregnant adults: A1C below 7%, 80 to 130 mg/dL (4.4 to 7.2 mmol/L) before meals, and below 180 mg/dL (10.0 mmol/L) at the 1 to 2 hour peak after a meal. The ADA stresses these are starting points: your clinician may set a lower or higher goal depending on age, other conditions, and your risk of low blood sugar.
A1C reflects your average over roughly two to three months. The ADA's conversion table shows how A1C maps to average glucose: 7% is about 154 mg/dL (8.6 mmol/L), 8% about 183 (10.2), 9% about 212 (11.8) and 10% about 240 (13.4). These are estimates with a wide range for each person.
| Reading | What it generally means | What to do |
|---|---|---|
| 80–130 mg/dL before meals; under 180 after (4.4–7.2; under 10.0) | Within ADA goals for many adults with diabetes | Keep going and keep screening |
| Above 180 mg/dL (10.0) often | Above target; symptoms may start around 180–200 | Review food, activity, medicine and illness with your care team |
| Above 180 for over a week, or two readings in a row above 300 (16.7) | Needs medical review | Call your doctor (Cleveland Clinic Abu Dhabi patient guidance) |
| Staying above 240 mg/dL (13.3) | Possible ketones building up | Check ketones and call your care team (Mayo Clinic) |
| Above 240 with ketones, or any high reading with vomiting, confusion or trouble breathing | Possible DKA | Emergency care |
| Extremely high, often above 600 mg/dL (33.3), without ketones | Possible hyperosmolar state (HHS) | Emergency care |
Thresholds for calling a doctor vary slightly between organisations. If your care team gave you personal numbers or a written sick-day plan, follow that first.
The Science How High Blood Sugar Damages the Body
Imagine your smallest blood vessels as thin garden hoses that feed your eyes, kidneys and nerves. When glucose stays high for years, the walls of those hoses are repeatedly injured. The National Eye Institute explains that in the retina this makes blood vessels blocked, leaky or prone to bleeding, and the eye responds by growing fragile new vessels that do not work well. The kidneys' filters and the vessels feeding nerves are affected in a similar way.
Clinicians divide the damage into two groups. Microvascular complications affect the smallest vessels: diabetic eye disease (retinopathy), kidney disease and nerve damage. Macrovascular complications affect larger arteries: heart attack, stroke and poor circulation in the legs.
Researchers have described several overlapping processes behind this, including glucose attaching to proteins (glycation), oxidative stress and chronic inflammation. These are simplified here, and the key practical point is the one the clinical trials show: the lower and steadier your average glucose, the lower your risk. In the landmark Diabetes Control and Complications Trial, better glucose control was associated with 50 to 76 percent fewer new and worsening eye, nerve and kidney complications in type 1 diabetes, and the benefit lasted for decades afterwards (the so-called legacy effect). Long-term follow-up of type 2 diabetes trials showed similar lasting benefits from early control. This is the ADA's summary of that evidence.
Two things make damage faster: high blood pressure and high cholesterol travelling alongside high glucose. This is why managing all three, not just sugar, is the standard of care.
Three Factors That Set Your Risk
Three variables explain most of the difference between people who develop complications and people who do not.
Signs Warning Signs of Long-Standing High Blood Sugar
These are the signs listed by Mayo Clinic and Cleveland Clinic. They tend to develop slowly, over days or weeks, and they overlap with many other conditions, so they are a reason to get tested, not a diagnosis.
System 1 Heart and Blood Vessels
The NIDDK reports that adults with diabetes are nearly twice as likely to have heart disease or a stroke as adults without diabetes, and that people with diabetes tend to develop heart disease at a younger age. Over time, high blood glucose can damage your blood vessels and the nerves that control your heart. Many people with diabetes also have high blood pressure or high cholesterol, which adds to the danger.
Importantly, the ADA notes that blood glucose is a weaker independent cardiovascular risk factor than blood pressure and cholesterol. That is why heart protection in diabetes means treating all three together, and why certain diabetes medicines (SGLT2 inhibitors and GLP-1-based medicines) are recommended in people with established heart disease, kidney disease, heart failure or obesity, independent of A1C. Whether one is right for you is a decision for your clinician.
What the NIDDK Recommends to Lower Heart and Stroke Risk
- +Stop smoking or using other tobacco products.
- +Manage your A1C, blood pressure and cholesterol. The ADA's common blood pressure goal for diabetes is below 130/80 mmHg, individualised by your clinician.
- +Be more physically active and find ways to manage stress.
- +Take medicines as prescribed, including those for blood pressure and cholesterol.
System 2 The Kidneys
The NIDDK explains that high blood glucose can damage the blood vessels in your kidneys. Most people with diabetic kidney disease have no symptoms, so the only way to know is to have your kidneys checked. Health professionals check your urine for albumin (a protein that leaks when filters are damaged) and use a blood test to see how well your kidneys filter. High blood pressure speeds the damage, which is why blood glucose and blood pressure goals are the two best levers to slow or prevent it.
The ADA and the National Kidney Foundation recommend screening every adult with diabetes once a year with both tests: the urine albumin-to-creatinine ratio (UACR) and the estimated glomerular filtration rate (eGFR). A raised UACR can show up about a decade before eGFR drops, so it is the earliest warning. A single raised result is usually repeated before a diagnosis is made.
Your kidneys filter your blood all day through millions of tiny vessels. High glucose and high pressure slowly make the filter leaky. First a little protein slips through, and later the filter cannot clean the blood well. Catching the leak early, with a simple urine test, is the difference between slowing the process and treating kidney failure.
Ask your clinician before regularly using over-the-counter anti-inflammatory painkillers such as ibuprofen or naproxen if you have any kidney problem, because they can reduce blood flow to the kidneys.
System 3 The Eyes
According to the National Eye Institute, diabetic retinopathy is the most common cause of vision loss for people with diabetes. Your risk rises the longer you have diabetes, and over time more than half of people with diabetes will develop it. In the early stages there are usually no symptoms. Later, blood vessels can bleed into the eye, producing dark floating spots or streaks like cobwebs, and scarring can cause retinal detachment. Diabetes also makes you 2 to 5 times more likely to develop cataracts and nearly doubles your risk of open-angle glaucoma. About 1 in 15 people with diabetes will develop diabetic macular edema, which blurs central vision.
The NEI advises a comprehensive dilated eye exam at least once a year. If retinopathy is found, treatment (anti-VEGF injections, laser treatment or surgery) can stop vision from getting worse, and anti-VEGF drugs can slow or even reverse diabetic retinopathy, but treatment will not undo vision damage that has already occurred. Keeping glucose, blood pressure and cholesterol in range lowers your risk. If you have diabetes and become pregnant, have a dilated eye exam as soon as possible.
Book your dilated eye exam today if you cannot remember your last one. Regular glasses tests do not replace it, because the eye doctor needs to widen your pupils to see the retina.
System 4 Nerves and Feet
The NIDDK says that about one-third to one-half of people with diabetes have peripheral neuropathy, nerve damage that typically affects the feet and legs and sometimes the hands and arms. High glucose, and high blood fats such as triglycerides, can injure the nerves over time. A different form, autonomic neuropathy, affects nerves that control body systems and can cause hypoglycemia unawareness, meaning you no longer feel the warning signs of a low.
The CDC explains the danger: nerve damage puts you at risk for foot ulcers, and a foot ulcer can get infected and not heal well. Slow-healing sores are also one of the classic signs of high blood sugar. The good news is that most serious foot complications can be prevented with a few minutes of daily care.
The Daily Foot Routine (CDC)
- +Look at both feet every day, even if they feel fine. Check for cuts, redness, swelling, sores, blisters, corns, calluses and nail changes. Use a mirror for the soles, or ask someone to help.
- +Wash in warm, not hot, water and do not soak your feet.
- +Get your feet checked at every primary-care visit, and see a foot doctor (podiatrist) every year, more often if you have nerve damage.
- +See a clinician promptly for any sore, blister or cut that is red, swollen, draining, or not clearly improving.
System 5 Brain and Memory
A 2024 meta-analysis of studies published between 2012 and 2023 found that people with diabetes had a relative risk of dementia of 1.59 compared with people without diabetes (about 59 percent higher). An earlier pooled analysis of prospective studies (Gudala and colleagues, 2013) found about a 73 percent higher risk of dementia overall, 56 percent for Alzheimer's disease and 127 percent for vascular dementia. Those numbers describe populations, not individuals, and the studies varied widely.
Part of the link is vascular: the small vessels of the brain are damaged by the same processes as the eyes and kidneys. The ADA also notes that cognitive impairment and severe hypoglycemia have a two-way relationship, so very low blood sugar matters as well as high. For that reason the ADA recommends regularly assessing cognitive function in people with diabetes.
System 6 Emergencies: DKA and HHS
The ADA's 2026 Standards of Care describe diabetic ketoacidosis (DKA) and the hyperglycemic hyperosmolar state (HHS) as serious, acute, life-threatening emergencies, and report that rates have increased in both type 1 and type 2 diabetes. DKA happens when the body lacks enough insulin, burns fat for fuel and produces ketones that make the blood acidic. It is most common in type 1 diabetes but can occur in anyone with diabetes. HHS involves extremely high glucose, often above 600 mg/dL (33.3 mmol/L), without significant ketones, and severe dehydration. The ADA says suspected HHS should be evaluated immediately in hospital.
Illness is the most common trigger, which is why the ADA recommends that people at risk of DKA have a written sick-day plan and ketone tests at home. The ADA says to measure ketones when you have symptoms or a precipitating factor such as illness or missed insulin, particularly if glucose exceeds 200 mg/dL (11.1 mmol/L). Mayo Clinic gives 240 mg/dL (13.3 mmol/L) as the level at which staying high needs attention and a ketone check. DKA can also occasionally happen without very high glucose, for example in pregnancy, and the ADA lists SGLT2 inhibitors, very-low-carbohydrate diets, prolonged fasting, dehydration and heavy alcohol intake as added risk factors, so never rely on the glucose number alone.
Seek Immediate Medical Attention If (ADA 2026)
- !You cannot tolerate drinking fluids, or vomiting continues.
- !Glucose or ketone levels are not improving with your usual insulin plan.
- !You feel confused, unusually drowsy, or generally worse.
Hope What Can Improve, and What May Not
What the Evidence Supports
- +Prevention works best. In the Diabetes Prevention Program, adults at high risk who aimed for about 7 percent weight loss and 150 minutes of brisk activity a week cut their chance of developing diabetes by 58 percent compared with placebo. The CDC says its prevention programme can lower risk by as much as 58 percent (71 percent over age 60).
- +Early control has a lasting legacy. The ADA notes that a finite period of intensive glucose lowering early on produced benefits that lasted for decades afterwards in type 1 and type 2 diabetes trials.
- +Eyes: treatment can stop retinopathy from worsening, and some anti-VEGF drugs can slow or reverse it, but lost vision does not come back (NEI).
- +Kidneys: glucose and blood pressure control slow or prevent progression. Medicines such as ACE inhibitors or ARBs are used when albumin in the urine is raised.
- +Nerves: keeping glucose on target is the best defence against neuropathy. Established nerve damage may not fully recover, so the goal is to prevent worsening and manage symptoms.
Action Plan 6 Steps to Take Now
- 1Find out where you stand. If you have never been tested, or your last test was more than a year ago, ask for an A1C and a fasting glucose. A1C reflects the past two to three months and does not require fasting. Abnormal results are confirmed with a repeat test before a diagnosis is made. A home meter is useful for monitoring but is not used to diagnose diabetes. Also ask for a kidney check (eGFR and UACR), a cholesterol panel and a blood pressure reading. If you have symptoms such as extreme thirst or heavy urination, do not wait: get tested within days.
- 2Agree your personal targets. The ADA's general goals are A1C below 7%, 80 to 130 mg/dL (4.4 to 7.2 mmol/L) before meals and below 180 mg/dL (10.0 mmol/L) after meals, but your clinician may set a different goal for you. Write your numbers down and put them on your phone. Re-test A1C at least twice a year, or about every three months if you are not at goal or your treatment changed (ADA).
- 3Handle a high reading safely today. Recheck to confirm the number. Drink water or other unsweetened, non-alcoholic fluids. Take your medicines as prescribed and do not skip doses. Look for a cause: illness, stress, missed medicine, a large meal or inactivity (all listed by Mayo Clinic). If you use insulin or have type 1 diabetes, check ketones when your glucose is high or you feel unwell, and ask your care team before vigorous exercise if ketones are present. Call your doctor if glucose stays high despite treatment, and go to emergency care using the red-flag list above.
- 4Build habits that lower your average. Aim for 150 minutes of moderate activity a week, such as brisk walking, plus muscle-strengthening work on two days. Build plates around non-starchy vegetables, protein or legumes, healthy fats and high-fibre carbohydrates, and cut back on sugary drinks and refined starches. Even modest weight loss helped in the Diabetes Prevention Program. For timing tips, see our evidence review of morning vs evening workouts for diabetics. Check with your clinician before starting a new exercise or eating plan, especially if you use insulin or a sulfonylurea.
- 5Never skip the screening schedule. Complications are caught early only by testing: A1C, kidney tests, a dilated eye exam, a foot exam and blood pressure checks. The table below shows how often.
- 6Review your whole risk, not just glucose. Ask your clinician about blood pressure and cholesterol targets, whether you should be on medicines that also protect the heart and kidneys, and how to stop smoking if you smoke. Stress and illness can push glucose up, so ask for a sick-day plan. Repeated high readings are a signal to adjust treatment, not a personal failure.
Table Your Screening Schedule
| What is checked | Test | How often | Source |
|---|---|---|---|
| Average glucose | A1C (and/or CGM data) | At least twice a year; about every 3 months if not at goal or treatment changed | ADA Standards 2026 |
| Kidneys | UACR (urine) + eGFR (blood) | Every year | ADA and National Kidney Foundation guidance |
| Eyes | Comprehensive dilated eye exam | At least once a year (more often if retinopathy is found) | National Eye Institute |
| Feet | Daily self-check; foot doctor exam | Self-check daily; professional exam yearly, more often with nerve damage | CDC |
| Heart risk | Blood pressure, cholesterol | At every routine visit; follow your clinician's schedule | NIDDK, ADA |
| Memory and thinking | Cognitive screen | Regularly, especially in older adults | ADA Standards 2026 |
Your clinician may set a different schedule based on your risk. See References for sources.
Avoid 6 Mistakes to Avoid
- Waiting for symptoms. Eye, kidney and nerve damage can be silent. Test on schedule.
- Stopping or changing medicine because you feel fine, or holding insulin when you are ill, without talking to your care team.
- Diagnosing yourself from one home reading. One high number may reflect illness, stress or a large meal. Ask for laboratory tests.
- Skipping the eye, kidney and foot checks because nothing hurts.
- Soaking your feet or ignoring a small sore. The CDC advises against soaking and recommends daily checks.
- Crash dieting or prolonged fasting while on insulin, a sulfonylurea or an SGLT2 inhibitor without medical guidance. The ADA lists fasting and very-low-carbohydrate diets as risk factors for DKA and a trigger for lows.
Key Takeaways: What Happens When Blood Sugar Stays High
Damage is cumulative. Risk rises with how high your glucose runs and how long it stays there, so testing early and acting early matters most.
Most complications start silently. No symptoms does not mean no damage to your kidneys, eyes or nerves.
Adults with diabetes are nearly twice as likely to have heart disease or stroke, so manage blood pressure and cholesterol alongside glucose.
Use the screening schedule: A1C, a yearly kidney check (eGFR and UACR), a yearly dilated eye exam, a daily foot check and a yearly foot exam.
Vomiting, confusion, trouble breathing, or glucose above 240 mg/dL (13.3 mmol/L) with ketones are emergencies. Get a written sick-day plan.
You can prevent and slow most complications, and early control has a lasting benefit. Some established damage may not fully reverse, which is why earlier is better.
Myths Common Misconceptions
FAQ Frequently Asked Questions
How long does blood sugar have to stay high to cause damage?
What blood sugar level is dangerous?
Is a reading of 200 mg/dL (11.1 mmol/L) too high?
Can high blood sugar cause damage if I do not have diabetes?
Can the damage from high blood sugar be reversed?
Which tests should I ask my doctor for?
Does high blood sugar cause dementia?
Know Your Numbers. Protect Your Organs.
Knowing your A1C, kidney and eye status is the first step. Join the VitaHive newsletter for new evidence-based guides on blood sugar, insulin resistance and metabolic health, delivered free.
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Sophia Williams, MS Clinical Nutrition, CDCES
Nutritionist with an MS in Clinical Nutrition and a CDCES certification, with experience since 2015 in diabetes management and weight loss. At VitaHive Health, she writes nutrition and food education content.
View Full Profile →Medical Disclaimer: This article is intended for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. The information provided is not a substitute for professional medical advice from a qualified healthcare provider. Always consult your physician, registered dietitian, or certified diabetes care and education specialist before changing your diet, exercise or medication, especially if you have diabetes, prediabetes, cardiovascular disease, kidney disease, or take insulin or other glucose-lowering medicines. Targets and screening schedules are general and may be different for you. If you have symptoms of very high blood sugar such as vomiting, confusion, fruity-smelling breath or trouble breathing, seek emergency medical care immediately. Never adjust your insulin or medicines without medical guidance.
“What Happens When Blood Sugar Stays High for Too Long,” VitaHive Health, 2026
