When blood sugar drops below about 70 mg/dL, your body launches a stress response that produces a cascade of symptoms, from shaking and sweating to confusion and, in severe cases, seizures or loss of consciousness. The effects range from mildly uncomfortable to life-threatening depending on how low glucose falls and how quickly you can correct it.
Your Body’s First Warning Signs
The earliest symptoms of low blood sugar come from your nervous system firing off alarm signals. These warning signs tend to hit fast: trembling hands, a pounding heart, sudden sweating, waves of anxiety or hunger, and nausea. Some people also feel tingling in their lips or fingertips. These are your body’s way of telling you to eat something, and they’re driven by a surge of adrenaline.
If blood sugar continues to fall, a second set of symptoms appears. These come from the brain itself running low on fuel. Glucose is the brain’s primary energy source, and even a modest deficit causes poor concentration, confusion, weakness, drowsiness, blurred vision, headaches, dizziness, and slurred speech. At this stage, thinking clearly enough to help yourself becomes harder, which is part of what makes low blood sugar dangerous.
What Happens Inside Your Body
The moment your blood sugar starts declining, your body activates a hormonal rescue system. Insulin secretion drops, and four counterregulatory hormones ramp up: glucagon, adrenaline, cortisol, and growth hormone. Each plays a distinct role. Glucagon signals the liver to release its stored glucose. Adrenaline does the same while also suppressing further insulin release, which is why you feel shaky and anxious. Cortisol shifts your metabolism toward producing new glucose from non-sugar sources and generating ketones as backup fuel. Growth hormone increases fat breakdown and makes cells temporarily less responsive to insulin, helping keep whatever glucose is available in the bloodstream longer.
This system works well in most people. But in those who experience frequent low blood sugar episodes, particularly people with diabetes on insulin or certain medications, the system can become blunted over time. The hormonal response weakens, and the warning signs arrive later or not at all.
Severe Low Blood Sugar: Seizures and Unconsciousness
When blood sugar drops to around 50 mg/dL or lower, the brain’s energy supply becomes critically compromised. Without adequate glucose, the pumps that maintain the electrical balance across nerve cell membranes start to fail. Calcium floods into cells, and levels of the brain’s main calming chemical drop while excitatory chemicals like glutamate spike. The result is neuronal hyperexcitability, which can trigger seizures.
If glucose isn’t restored, the progression continues toward loss of consciousness and coma. Prolonged severe episodes can cause brain cell death. This is a medical emergency. The brain cannot wait for a slow recovery; it needs glucose immediately.
Effects on the Heart
Low blood sugar doesn’t just affect the brain. It also disrupts heart rhythm. During hypoglycemic episodes, the heart’s electrical recovery phase (measured as the QT interval on an ECG) becomes prolonged. This prolongation is a known risk factor for dangerous irregular heart rhythms and sudden cardiac death. The risk is particularly elevated during nighttime episodes, when studies have found a strong correlation between low blood sugar and abnormal heart rhythms in sleeping patients. This connection exists regardless of whether someone has diabetes.
Nighttime Episodes Are Especially Risky
Nearly half of all low blood sugar episodes, and more than half of severe episodes, happen during sleep. That’s a problem because you can’t recognize symptoms while unconscious. Signs that a nighttime episode is occurring include restless or irritable sleep, sweaty or clammy skin, trembling, sudden changes in breathing rate, nightmares, and a racing heartbeat. A bed partner may notice these signs before the person experiencing them does.
Nighttime lows are dangerous precisely because they can go undetected long enough for blood sugar to reach levels that cause seizures, cardiac rhythm disturbances, or both.
When Warning Signs Disappear
One of the most concerning complications of repeated low blood sugar is a condition called hypoglycemia unawareness. Normally, symptoms kick in at a predictable glucose level, giving you time to eat or drink something. But with repeated episodes, the threshold for feeling symptoms keeps dropping. If you used to feel shaky at 60 mg/dL, you might not notice anything until you hit 55, then 50, then lower. The critical problem: the glucose level that causes unconsciousness doesn’t drop along with it. The gap between “I feel fine” and “I’m unconscious” narrows until there’s almost no warning at all.
Risk factors for developing this unawareness include taking insulin or sulfonylurea medications, having diabetes for 20 to 30 years, targeting very aggressive blood sugar goals, and having conditions like dementia, anxiety, or depression that make diabetes management harder. According to the National Institute of Diabetes and Digestive and Kidney Diseases, the good news is that carefully avoiding low blood sugar episodes for several weeks can partially restore awareness by resetting the symptom threshold upward.
Long-Term Cognitive Effects
A single mild episode of low blood sugar, quickly corrected, is unlikely to cause lasting harm. But repeated episodes are a different story. A large meta-analysis published in Frontiers in Neurology found that people with a history of hypoglycemia had a 47% higher likelihood of developing cognitive dysfunction compared to those without. The relationship follows a dose-response pattern: one episode raised the risk by 20%, two episodes by 41%, and three or more episodes by 62%. Each additional episode contributed roughly an 18% further increase in risk.
The mechanism behind this involves the same process that causes acute symptoms. When brain cells are repeatedly starved of energy, the resulting calcium overload and buildup of reactive oxygen species cause cumulative damage. Over years, this can accelerate cognitive decline and increase dementia risk, particularly in older adults with type 2 diabetes.
What a Severe Episode Looks Like for Others
If someone near you has severely low blood sugar, they may be unable to help themselves. They might appear confused, combative, or drunk. They may have a seizure or become unresponsive. For people with diabetes who are at risk of severe episodes, emergency glucagon is available as a kit that a bystander can administer. Traditional kits require mixing a powder with liquid and injecting it into the upper arm, thigh, or buttocks. Newer formats include nasal sprays and pre-filled auto-injectors that require no mixing. After giving glucagon, call emergency services. Once the person is alert enough to swallow, give them a carbohydrate-rich food or drink to prevent their blood sugar from dropping again.
For mild to moderate lows where the person is still conscious and able to swallow, 15 to 20 grams of fast-acting carbohydrates (glucose tablets, juice, regular soda) will typically bring blood sugar back up within 15 minutes. Checking levels again after that window helps confirm recovery.
Who Is Most at Risk
Low blood sugar most commonly affects people taking insulin or medications called sulfonylureas and meglitinides, which stimulate the pancreas to release more insulin regardless of current blood sugar levels. The 2025 American Diabetes Association standards of care specifically recommend moving away from these medications when possible, favoring newer drug classes that don’t carry the same hypoglycemia risk. Continuous glucose monitors are now recommended for all people with diabetes, with an especially strong recommendation for those on insulin or other medications that can cause lows.
People without diabetes can also experience low blood sugar from prolonged fasting, excessive alcohol intake, certain rare tumors, or hormonal deficiencies, though these situations are far less common. In anyone, skipping meals combined with intense physical activity can drop glucose low enough to cause symptoms.