Low blood glucose, called hypoglycemia, happens when your blood sugar drops below the level your body needs to function normally. For people with diabetes, that threshold is generally below 70 mg/dL. For people without diabetes, it’s typically below 55 mg/dL. The causes range from medication side effects and missed meals to hormonal imbalances and rare tumors, and understanding which one applies to you is the key to preventing it from happening again.
How Your Body Normally Keeps Glucose Stable
Your body runs a tightly controlled system to keep blood sugar within a narrow range. When levels start to drop, your pancreas releases glucagon, a hormone that signals the liver to break down its stored sugar (glycogen) and release it into the bloodstream. If glucose keeps falling, your adrenal glands release stress hormones like epinephrine and cortisol. Epinephrine speeds up the liver’s glucose release. Cortisol reduces how much glucose your muscles pull from the blood and ramps up the liver’s ability to manufacture new glucose from scratch. Growth hormone pitches in by making your cells temporarily less responsive to insulin, which keeps more sugar circulating.
When any part of this chain fails, whether from medication, disease, or hormonal deficiency, glucose can drop to dangerous levels.
Diabetes Medications Are the Most Common Cause
The single most frequent trigger for low blood sugar is diabetes medication, particularly drugs that push the pancreas to release more insulin regardless of what your blood sugar is doing at that moment. Sulfonylureas (glipizide, glimepiride, glyburide) and glinides (nateglinide, repaglinide) are the biggest culprits in this category. Insulin injections carry the same risk if the dose is too high, mistimed, or not matched to food intake.
Other diabetes drugs don’t usually cause low glucose on their own but can when combined with sulfonylureas. Metformin, SGLT2 inhibitors, and thiazolidinediones all fall into this group. The combination effect matters: if you’re on multiple diabetes medications, your risk is higher than on any single one.
Timing plays a big role. Taking your medication and then skipping a meal, eating less than usual, or exercising more than expected can all tip the balance. The medication keeps working even when there isn’t enough incoming food to offset it.
Skipping Meals, Fasting, and Not Eating Enough
Your liver stores enough glycogen to maintain blood sugar for roughly 12 to 24 hours, depending on your activity level and how well-stocked those reserves are. After that, your body relies on manufacturing glucose from amino acids and other raw materials. If you’re fasting, dieting heavily, or simply forgot to eat, your glucose can drop once those liver stores run out. This is especially true if you’re also exercising, drinking alcohol, or taking medications that lower blood sugar.
People who eat very low-carb diets sometimes experience mild dips, though the body usually adapts by shifting to fat-based fuel. The real danger is prolonged fasting combined with another risk factor.
Alcohol and Low Blood Sugar
Alcohol is one of the more underappreciated causes of hypoglycemia. When your liver is busy processing alcohol, it can’t do its other critical job: producing new glucose. Research shows that alcohol causes a prompt and significant drop in the liver’s glucose output, as much as 65% in some studies. This effect is most dangerous when your glycogen stores are already low, which happens if you’ve been fasting, haven’t eaten in several hours, or have been exercising.
The timing is deceptive. You might feel fine while drinking and then experience a glucose crash hours later, sometimes in the middle of the night. For people with diabetes, alcohol-related hypoglycemia can be severe because the liver suppression stacks on top of medication effects.
Reactive Hypoglycemia After Meals
Some people experience low blood sugar not from fasting but from eating. Reactive hypoglycemia, sometimes called postprandial hypoglycemia, causes a glucose drop within four hours after a meal. It tends to happen after eating high-sugar or high-carbohydrate foods that trigger a large insulin spike. The insulin overshoots, pulling blood sugar down past where it should be.
In people without diabetes, the exact cause often isn’t clear. People who’ve had gastric bypass or other bariatric surgery are particularly prone to it because food enters the small intestine faster, triggering a more dramatic insulin response. Certain inherited metabolic disorders and, rarely, tumors can also cause it.
Hormonal Deficiencies
Because your body relies on multiple hormones to raise blood sugar when it drops, a deficiency in any of them can cause hypoglycemia. Adrenal insufficiency, where the adrenal glands don’t produce enough cortisol, removes one of your body’s main glucose-boosting mechanisms. Pituitary gland disorders can reduce growth hormone production, which also plays a role in glucose regulation. In children, growth hormone deficiency is a more common cause of hypoglycemia than in adults.
These hormonal causes are relatively uncommon, but they’re worth investigating if you experience repeated low blood sugar episodes without an obvious trigger like medication or fasting.
Liver and Kidney Disease
Your liver is the primary organ responsible for storing and releasing glucose. Severe liver disease from hepatitis, cirrhosis, or other conditions can impair this function enough to cause hypoglycemia. Your kidneys also contribute to glucose production, particularly during prolonged fasting. Kidney failure reduces this backup supply, making low blood sugar more likely.
Insulinoma and Other Rare Causes
An insulinoma is a small tumor in the pancreas that produces insulin continuously, regardless of your blood sugar level. It’s rare, but it’s one of the few conditions that causes repeated, unexplained hypoglycemia in people who don’t have diabetes. Diagnosis typically involves a supervised fast in a hospital, lasting up to 72 hours, during which doctors monitor blood sugar and insulin levels. Very low glucose paired with inappropriately high insulin points to an insulinoma. Most insulinomas are benign and can be surgically removed.
Exercise and Delayed Glucose Drops
Physical activity burns through glucose, and for most people the body compensates by releasing stored sugar and adjusting insulin levels. But for people with diabetes, this compensation system doesn’t work perfectly. Exercise can cause blood sugar to drop during the activity itself, and it can also cause delayed hypoglycemia several hours afterward as muscles continue pulling in glucose to replenish their own stores.
This delayed effect is particularly tricky because it can happen at night after an afternoon or evening workout, when you’re asleep and less likely to notice symptoms. The risk is highest after unusually intense or prolonged exercise, especially if insulin or other glucose-lowering medications haven’t been adjusted.
How Severity Is Classified
Not all low blood sugar episodes are equal. The American Diabetes Association breaks hypoglycemia into three levels. Level 1 is a glucose reading below 70 mg/dL but at or above 54 mg/dL. You’ll likely feel early warning signs like shakiness, sweating, and hunger, and you can treat it yourself with fast-acting carbohydrates. Level 2 is below 54 mg/dL, where symptoms become more serious and can include confusion, blurred vision, and difficulty concentrating. Level 3 is any episode severe enough that you need someone else’s help to recover, regardless of the specific glucose number. This can involve seizures or loss of consciousness.
When Warning Signs Disappear
One of the most dangerous aspects of recurring low blood sugar is that your body can stop warning you it’s happening. About 31% of people with type 1 diabetes have impaired awareness of hypoglycemia, meaning they no longer feel the early symptoms like shakiness and sweating. This happens because repeated low glucose episodes dull the body’s hormonal alarm system. People with type 1 diabetes also tend to lose their glucagon response early in the disease, which creates what researchers describe as a “perfect storm”: insulin can still push glucose down, but the main hormone responsible for pushing it back up no longer responds.
The result is that blood sugar can plummet to dangerous levels before the person realizes anything is wrong. Continuous glucose monitors, which alert you to dropping levels before symptoms would normally appear, have become an important tool for people in this situation.