What is a dangerous blood sugar level?

"Dangerous" blood sugar has two ends. Glucose that falls too low starves the brain within minutes; glucose that stays too high poisons the body more slowly but can tip into crises that are just as lethal. The numbers that mark each edge are worth knowing, because at both extremes the safe response is treatment, not waiting.

Too low: below 70 mg/dL

Low blood glucose, or hypoglycaemia, mostly affects people whose diabetes is treated with insulin or certain other glucose-lowering drugs. The national diabetes institute sets the alert threshold at a blood glucose reading lower than 70 mg/dL, the point at which a person should treat it and recheck [s1]. Early warning signs — shakiness, sweating, hunger, a racing heart, irritability — come from the body's adrenaline response and are the cue to act.

There is a lower line as well. A joint position statement from the American Diabetes Association and the European Association for the Study of Diabetes established that glucose concentrations of less than 3.0 mmol/L, equal to 54 mg/dL, are serious and clinically important enough to be reported in trials [s2]. Below that level the risk shifts from unpleasant symptoms to impaired thinking and consciousness.

The most dangerous form is severe hypoglycaemia, which the institute defines not by a number but by its effect: blood glucose drops so low that a person can no longer treat it themselves [s1]. At that stage it can cause confusion, unconsciousness, seizures and, if untreated, death [s1]. This is why the standard advice is to treat readings under 70 mg/dL promptly with fast-acting carbohydrate rather than to see how far they fall.

Too high: the point where it becomes a crisis

High blood sugar is more insidious because it does not hurt in the moment. Sustained hyperglycaemia damages blood vessels over years, but it can also escalate into two acute emergencies, and here the numbers are specific.

The first is diabetic ketoacidosis. In the 2024 consensus report on hyperglycaemic crises — assembled by the ADA with European, British and other professional bodies — ketoacidosis is diagnosed by three findings together: a blood glucose above 200 mg/dL (11.1 mmol/L), or a prior history of diabetes regardless of the presenting glucose; blood beta-hydroxybutyrate, a ketone, at 3.0 mmol/L or higher; and evidence of acidosis, meaning a venous pH below 7.3 or a bicarbonate below 18 mmol/L [s3]. Note that the glucose figure is not especially high — ketoacidosis is defined by the acid and ketones, not by an extreme sugar reading, which is why it can occur at glucose levels people might otherwise dismiss.

The second is the hyperglycaemic hyperosmolar state, which sits at the opposite end of the glucose scale. The consensus report diagnoses it by a blood glucose above 600 mg/dL (33.3 mmol/L), an effective serum osmolality above 320 mOsm/kg, a venous pH above 7.30 and a bicarbonate above 18 mmol/L [s3]. Here the danger is extreme concentration of the blood and profound dehydration rather than acid build-up.

These are not always separate. The report notes that clinical overlap between ketoacidosis and the hyperosmolar state has been reported in more than one-third of people with hyperglycaemic crises [s3].

What the numbers do and do not tell you

A few points keep these figures in proportion. The 70 and 54 mg/dL thresholds are general alert levels; a person's own target range, set with their care team, may differ, and the institute frames 70 mg/dL as the trigger to treat rather than a fixed emergency line [s1]. The ketoacidosis and hyperosmolar criteria, by contrast, are diagnostic definitions that require blood tests beyond a home glucose meter — ketones, pH and osmolality — so a glucose reading alone cannot confirm or rule them out [s3].

That last detail is the practical heart of it. A single high number on a home meter is a reason to check ketones and seek advice if unwell, not a diagnosis in itself. And the symptoms that accompany these crises — nausea, vomiting, abdominal pain, deep or rapid breathing, extreme thirst, drowsiness or confusion — matter as much as the reading.

When to seek help

At the low end, treat readings under 70 mg/dL at once and get emergency help for anyone who is confused, unconscious or seizing [s1]. At the high end, vomiting, laboured breathing, severe dehydration or altered awareness in someone with high glucose is an emergency, because it may signal a crisis whose full diagnosis depends on tests only a clinic can run [s3]. Both edges of the scale are dangerous, and both are treatable when caught early.