You wake up with a dry mouth that won’t quit. By mid-morning, your water bottle is empty again, and you’ve already taken two bathroom trips. Later, your vision goes a little blurry, like you’re looking through a smudged window. Is it just a weird day, or is something going on?
High blood sugar symptoms can creep in slowly over days or weeks, or show up fast during illness, stress, or missed meds. The tricky part is that some people feel “fine” even when their glucose is running high, until it isn’t.
This guide breaks down the most common warning signs, the glucose numbers that usually raise concern, and the practical difference between calling your doctor, using urgent care, or going to the ER. The goal is calm and clear, so you can act early and avoid the scary complications that come from waiting too long.
High blood sugar symptoms that should get your attention
High blood sugar (also called hyperglycemia) happens when there’s not enough insulin, insulin isn’t working well, or your body’s needs suddenly change. A missed dose, a pump problem, a big meal, or a stressful week can be enough to tip things upward. So can infections, surgery, and certain meds (steroids are a common example). The American Diabetes Association’s overview of hyperglycemia causes lays out these triggers in plain terms.
Symptoms often build because extra glucose doesn’t just sit quietly in the blood. It pulls fluid with it, like sugar drawing water into a sticky syrup. That shift can leave you dehydrated and drained, even if you’re drinking more than usual.
Also important: some people have very few symptoms, especially early on. That’s why prediabetes signs can be easy to miss, and why a simple test can matter more than “how you feel.”
The most common signs people notice first
The classic pair is frequent urination and thirst. When glucose rises past what your kidneys can handle, extra sugar spills into urine and drags water along with it. You pee more, then you get thirsty, then you pee more. It can become a loop.
Along with that, many people notice:
- Dry mouth or waking up feeling “parched,” even after drinking the night before.
- Blurry vision that comes and goes. High glucose can change fluid levels in the eye and affect focus.
- Headaches, especially if you’re also dehydrated or not sleeping well because you’re up at night.
- Fatigue and blood sugar changes that feel like you’re moving through wet cement. Your cells can’t use glucose well without enough effective insulin, so you can feel worn out even if you ate.
- Increased hunger (sometimes right after eating) because your body is struggling to move fuel into cells.
- Unexpected weight loss can be a red flag, especially if you’re eating normally. If the body can’t use glucose well, it may break down fat and muscle for energy.
A short real-life scenario: someone who usually sleeps through the night starts waking up three times to pee. They’re finishing a glass of water each time, and by morning they feel wrung out and foggy. If that pattern repeats, it’s worth checking a glucose reading and calling a clinician, even if there’s no pain.
For a symptom-focused medical summary, see Cleveland Clinic’s hyperglycemia symptoms and treatment.
Less obvious symptoms that still matter
Not all high blood sugar symptoms scream “blood sugar.” Some look like everyday annoyances, until you notice they’re piling up.
Common “quieter” signs include:
- Slow-healing cuts or sores that linger longer than usual.
- More infections, including yeast infections and urinary tract infections (UTIs). Extra glucose can make it easier for certain germs to grow.
- Itchy or very dry skin, sometimes with more irritation than you’re used to.
- Tingling, burning, or numbness in hands or feet. Nerve issues can have many causes, but long-running high glucose is one of them.
- Mood or focus changes, like irritability, brain fog, or feeling unusually “off.”
Any one of these can come from other causes (dry winter air, a new soap, a busy month). The signal gets stronger when several show up together, or when they repeat. If you’re seeing a cluster of these plus thirst, peeing more, or fatigue, that’s a good moment to check your glucose or schedule labs.
For a broad, trustworthy overview of hyperglycemia and related issues, MedlinePlus on high blood sugar is a solid reference.
What glucose numbers mean, and glucose levels when to worry
Symptoms matter, but numbers help you make decisions. Glucose can be checked with:
- A fingerstick meter, which measures glucose in a drop of blood.
- A continuous glucose monitor (CGM), which estimates glucose from fluid under the skin. CGMs are great for trends, but they can lag behind rapid changes, like right after eating or exercising.
You’ll hear three main types of readings:
- Fasting glucose: usually measured after at least 8 hours without calories (often first thing in the morning).
- After-meal (post-meal) glucose: commonly checked 1 to 2 hours after eating.
- A1C: a lab test that reflects your average glucose over about 2 to 3 months.
Targets can differ based on age, pregnancy, other health conditions, and whether you’re at risk for low blood sugar. If you already have diabetes, your clinician’s plan comes first.
Quick guide to fasting glucose, after-meal readings, and A1C basics
If you’re trying to understand fasting glucose high results, these are commonly used cut points for diagnosis:
- Fasting glucose
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher (usually confirmed with repeat testing)
- A1C
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher (usually confirmed)
The CDC explains these ranges clearly in its page on the A1C test for diabetes and prediabetes. If you want a bit more detail on what A1C measures, Mayo Clinic’s A1C test overview is helpful.
After meals, glucose usually rises and then comes back down. What’s “too high” depends on the person, but many non-pregnant adults with diabetes are advised to aim for a peak below a certain level, often around the 180 mg/dL mark at 1 to 2 hours. Your clinician may set a different goal.
If you get a surprising number, don’t panic. Re-check smartly:
- Wash and dry your hands (food residue can falsely raise a fingerstick).
- Re-test in 10 to 15 minutes.
- Think about what changed: a high-carb meal, missed meds, less activity, poor sleep, stress, or an infection.
If you feel fine but the number is high, what to do in the next few hours
This is where people get stuck. You see a high reading, but you don’t feel terrible, so you wonder if it matters. It does, because repeated highs can snowball, especially during illness.
Here’s a practical, same-day plan that fits most situations (and keeps you from making risky moves):
- Recheck to confirm it’s real, and look at the trend if you use a CGM.
- Drink water if you’re allowed to (ask your clinician if you have kidney or heart issues that limit fluids).
- Move a little if it’s safe, like a gentle 10 to 20-minute walk. Skip intense workouts if you feel sick, dizzy, or you’ve been told to check ketones first.
- Avoid extra carbs while you’re troubleshooting. Stick to your usual meal plan.
- Take your prescribed meds as directed. Don’t double a dose “to catch up” unless your clinician has told you to.
- Check ketones if you’ve been advised, especially if you have type 1 diabetes, you’re sick, or your readings are very high. Some people are also told to check ketones when using certain diabetes meds.
- Track symptoms (nausea, stomach pain, rapid breathing, confusion, severe thirst). The pattern matters.
If readings stay high for several hours, or you keep spiking above your usual range for more than a day, contact your diabetes care team or primary care clinic the same day. Waiting it out is how mild hyperglycemia turns into a bigger problem.
When to seek help: call your doctor, use urgent care, or go to the ER
Think of your response like a thermostat. A small change needs a small adjustment, but a sudden heat surge needs fast action. Both the number and how you feel count.
If you’re pregnant, caring for a child, older, living alone, or you have type 1 diabetes, it’s smart to act sooner rather than later. If you’re newly diagnosed or don’t have supplies (no strips, no ketone tests, no working meter), treat that as a reason to seek help promptly.
Call your primary care doctor soon if symptoms keep coming back
Call within a few days (or sooner, if you’re worried) if you’re noticing a repeat pattern like:
- Ongoing thirst and frequent urination
- New fatigue that doesn’t match your routine
- Blurry vision that comes and goes
- Repeated yeast infections or UTIs
- Unexpected weight loss, or a big jump in hunger
- Home readings that are often higher than your usual range
This is often how prediabetes signs or early diabetes show up. A clinician can order labs (fasting glucose and A1C are common), review medications that may raise glucose (like steroids), and help you build a plan that fits your life.
Help your appointment go faster by bringing a short log for 3 to 7 days: glucose readings (with times), meals, meds, activity, sleep, and symptoms. You don’t need perfection, you need clues.
Get urgent help now for danger signs of very high blood sugar
Some symptoms mean your body is struggling to cope, and you need same-day urgent care or the ER.
Get urgent help right away if you have high glucose plus any of these red flags:
- Vomiting or you can’t keep fluids down
- Severe belly pain
- Deep or fast breathing
- Fruity-smelling breath
- Confusion, extreme sleepiness, or fainting
- Chest pain
- Signs of severe dehydration (very dry mouth, dizziness, weakness, not peeing much)
Also take very high readings seriously. As a practical example, a glucose reading around 300 mg/dL or higher that doesn’t improve with your usual correction plan, especially with symptoms, should prompt urgent evaluation. Two dangerous emergencies clinicians worry about are diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS). They can start with dehydration and nausea and escalate quickly.
If you suspect DKA or HHS, or you’re not sure what’s happening, don’t try to “sleep it off.” Call 911 or go to the ER. It’s better to be told it’s not an emergency than to arrive too late.
Conclusion
High blood sugar can whisper or shout. The key is noticing repeat patterns, recognizing high blood sugar symptoms, and checking a reading when you can. Pair the number with how you feel, then match your next step to the situation, from a routine clinic call to urgent care or the ER.
Don’t ignore symptoms that keep returning, even if they seem mild. Early care can prevent complications and make treatment simpler. If you’re due for a checkup, ask your doctor to run fasting glucose and A1C tests, and use your glucose hub to track results. If you already have diabetes, work with your clinician on a clear plan for what to do after a high reading. If serious symptoms come on quickly, go to the ER or call for emergency care.
Gas S. is a health writer who covers metabolic health, longevity science, and functional physiology. He breaks down research into clear, usable takeaways for long-term health and recovery. His work focuses on how the body works, progress tracking, and changes you can stick with. Every article is reviewed independently for accuracy and readability.
- Medical Disclaimer: This content is for education only. It doesn’t diagnose, treat, or replace medical care from a licensed professional. Read our full Medical Disclaimer here.