A blood glucose level below 70 mg/dL (3.9 mmol/L) is considered low and can cause harm. A level below 54 mg/dL (3.0 mmol/L) is a medical emergency requiring immediate action. These thresholds apply whether you have diabetes or not, though the causes and frequency of low blood sugar differ significantly between the two groups.
How Low Blood Sugar Feels
The symptoms of low glucose come in two waves, driven by different parts of your body’s alarm system. The first wave is your nervous system sounding the alert: sweating, shakiness, a racing heartbeat, anxiety, and sudden intense hunger. These early symptoms are your body telling you to eat something now. Most people recognize this stage and can treat it themselves.
If glucose keeps dropping, a second set of symptoms appears as your brain starts running short on fuel. These include weakness, dizziness, difficulty concentrating, confusion, blurred vision, and behavior changes that can look like intoxication to people around you. In extreme cases, this progresses to seizures, loss of consciousness, and coma. The shift from the first wave to the second can happen quickly, which is why acting on early symptoms matters so much.
Common Causes in People With Diabetes
The most frequent cause is a mismatch between insulin (or certain diabetes medications) and what your body actually needs at that moment. Taking too much insulin, skipping or delaying a meal after taking medication, or exercising more than usual without adjusting your dose can all push glucose below 70 mg/dL. Alcohol adds another variable because it interferes with your liver’s ability to release stored glucose, sometimes causing a delayed drop hours after drinking.
Low Glucose Without Diabetes
Low blood sugar in people without diabetes is less common but does happen. Several serious medical conditions can trigger it. Severe liver disease, including hepatitis and cirrhosis, impairs your liver’s ability to produce glucose. Advanced kidney disease can prevent your body from properly clearing certain medications, leading to a buildup that pushes glucose down. Severe infections and advanced heart disease are also recognized causes.
Prolonged starvation and malnutrition deplete your body’s glycogen stores, the backup supply your liver uses to maintain blood sugar between meals. Eating disorders, particularly anorexia nervosa, carry a real risk of hypoglycemia for this reason.
There’s also a form called reactive hypoglycemia, where blood sugar drops within four hours after eating. This typically happens when your body overproduces insulin in response to a meal, particularly one high in refined carbohydrates. The initial spike in blood sugar triggers a surge of insulin that overshoots, pulling glucose too low. If you notice symptoms like shakiness or lightheadedness a couple of hours after meals, this pattern is worth investigating.
The 15-15 Rule for Treatment
The standard approach for mild to moderate low blood sugar is simple: eat 15 grams of fast-acting carbohydrates, then wait 15 minutes and recheck your blood sugar. Good options include glucose tablets, 4 ounces of juice, or a tablespoon of honey. If your level is still below 70 mg/dL, repeat the cycle. Once it stabilizes, follow up with a small snack or meal to prevent another drop.
If someone loses consciousness or has a seizure from low blood sugar, they should not be given food or drink because of the choking risk. This is when emergency glucagon, a fast-acting injectable or nasal medication, becomes necessary. If you or someone you live with is at risk for severe episodes, keeping an emergency glucagon kit in an accessible place (like a bedside drawer) is a practical safeguard.
Low Blood Sugar During Sleep
Nocturnal hypoglycemia is particularly dangerous because you’re not awake to notice the early warning signs. Clues that it’s happening include restless or irritable sleep, waking up drenched in sweat, nightmares, trembling, or noticeable changes in breathing. A bed partner may notice these signs before you do.
If nighttime episodes are suspected, the typical next steps include adjusting the dose or timing of insulin, setting an early morning alarm to test blood sugar and track how often drops occur, or using a continuous glucose monitor that checks levels every five minutes and sounds an alarm when glucose falls too low. The last option is generally reserved for people who experience frequent or severe nighttime episodes.
Why Some People Stop Feeling Symptoms
One of the more dangerous complications of recurrent low blood sugar is something called hypoglycemia unawareness. Here’s how it works: each time your glucose drops low, the threshold at which your body triggers warning symptoms shifts a little lower. So if yesterday you felt shaky at 60 mg/dL, today you might not notice anything until you hit 55 mg/dL. The critical problem is that the glucose level triggering unconsciousness does not shift downward. The gap between “I feel fine” and “I’ve passed out” keeps narrowing.
This tends to affect people who have had diabetes for 20 or 30 years, those on insulin or sulfonylurea medications, and those who have been pushing aggressively for very low glucose targets. People with cognitive impairment, dementia, anxiety, or depression are also at higher risk because these conditions make it harder to recognize and respond to subtle changes in how they feel.
Long-Term Health Risks of Repeated Episodes
A single mild episode of low blood sugar, treated promptly, is not a major health concern. Repeated severe episodes are a different story. Prolonged hypoglycemia can cause brain injury and trigger dangerous heart rhythm disturbances. In people with type 2 diabetes, severe hypoglycemia is one of the strongest predictors of major cardiovascular events and death. Large clinical trials have found that people who experience one or more severe episodes face a significantly elevated risk of dying, with hazard ratios ranging from 1.67 to 4.28 depending on the study.
The mechanisms behind this increased risk involve a cascade of harmful changes: increased inflammation, a shift toward blood clotting, damage to blood vessel walls, and reduced blood flow to the heart. Studies estimate a mortality risk of 2% to 4% after a severe hypoglycemic episode in people with diabetes. These numbers underscore why preventing lows is just as important as preventing highs in blood sugar management.