Low blood sugar, or hypoglycemia, happens when glucose in your bloodstream drops below 70 mg/dL. The most common cause is diabetes medication, particularly insulin and a class of drugs called sulfonylureas, but plenty of non-diabetes causes exist too, from skipping meals to drinking alcohol to rare hormone disorders.
How Your Body Normally Controls Blood Sugar
Your blood sugar stays stable thanks to two hormones made in the pancreas that work like a seesaw. Insulin lowers blood sugar by moving glucose out of the bloodstream and into cells for energy. Glucagon raises it back up when levels dip too low, signaling the liver to convert its stored glucose into a usable form and release it into the blood. Glucagon also triggers your body to manufacture new glucose from other raw materials like amino acids.
When this system works well, blood sugar stays in a narrow range regardless of when you last ate. Low blood sugar happens when something disrupts this balance: too much insulin, not enough glucagon or other protective hormones, a liver that can’t release glucose properly, or simply not enough fuel coming in.
Diabetes Medications Are the Leading Cause
If you have diabetes, medication is by far the most likely reason for a low blood sugar episode. The biggest culprits are insulin (especially if the dose is too high or you skip a meal after taking it) and sulfonylureas like glipizide, glimepiride, and glyburide, which push your pancreas to produce more insulin regardless of what your blood sugar is doing at that moment.
Other diabetes drugs carry some risk as well, though usually only in combination with sulfonylureas. These include glinides (like nateglinide and repaglinide), certain newer glucose-lowering drugs, and the insulin-sensitizing class that includes pioglitazone and rosiglitazone. Metformin on its own rarely causes lows, but paired with a sulfonylurea it can. The risk goes up when you take these medications and then exercise more than usual, eat less than planned, or drink alcohol.
Skipping Meals and Prolonged Fasting
Your body stores a limited supply of glucose in the liver, typically enough to last roughly 12 to 24 hours depending on your activity level. When you skip meals or go long stretches without eating, that reserve gets depleted. If your body can’t manufacture new glucose fast enough to keep up with demand, blood sugar drops. This is especially likely if you’re also exercising, since muscles burn through glucose quickly.
For most healthy people, the body compensates well during short fasts. But certain conditions make the system less resilient: if you’re on medications that lower blood sugar, if your liver isn’t functioning well, or if you have a hormonal deficiency that weakens your body’s backup systems.
Alcohol’s Effect on Blood Sugar
Drinking alcohol is one of the most overlooked causes of low blood sugar. Your liver treats alcohol as a priority toxin and shifts its resources to processing it. While it’s busy doing that, its ability to produce new glucose drops sharply. Research published in the American Journal of Physiology found that gluconeogenesis (the liver’s process for making fresh glucose) decreased by 45% in the five hours after alcohol consumption, and overall glucose output from the liver fell by 12%.
This matters most when you drink on an empty stomach or after exercise, since your liver’s glucose reserves are already low. The risk is also higher hours after drinking, not just while you’re actively consuming alcohol, which is why people sometimes wake up with low blood sugar the morning after a night out. If you take insulin or sulfonylureas, alcohol amplifies the risk significantly.
Reactive Hypoglycemia After Meals
Some people experience blood sugar drops not from fasting but within a few hours of eating. This is called reactive (or postprandial) hypoglycemia, and it typically occurs within four hours after a meal. The pattern is that your body overproduces insulin in response to a surge of glucose, especially after high-carbohydrate meals, and the excess insulin drives blood sugar below normal.
Reactive hypoglycemia is more common in people who have had gastric bypass or other bariatric surgery, since food enters the small intestine faster and triggers a rapid, exaggerated insulin response. It can also result from inherited metabolic conditions or, rarely, certain types of tumors. For many people, though, the exact mechanism is never fully pinpointed. Eating smaller, more frequent meals that combine protein, fat, and complex carbohydrates rather than simple sugars often reduces episodes.
Hormone Deficiencies
Your body relies on several hormones beyond insulin and glucagon to keep blood sugar stable. Cortisol and growth hormone both play essential roles in glucose counterregulation: they increase the rate at which your liver produces glucose and they blunt the effects of insulin. When these hormones are deficient, your body loses a critical safety net against blood sugar drops.
Adrenal insufficiency, the condition where your adrenal glands don’t produce enough cortisol, is one recognized cause. Primary adrenal insufficiency (Addison disease) affects roughly 82 to 144 people per million in Western countries and tends to appear between ages 30 and 50. Secondary adrenal insufficiency, caused by problems with the pituitary gland, is actually more common (150 to 280 per million) and typically diagnosed in the 60s. Growth hormone deficiency, particularly in children, can also lead to episodes of low blood sugar during fasting.
Liver and Kidney Disease
Your liver is the main organ responsible for storing glucose and releasing it when blood sugar falls. Severe liver disease, whether from cirrhosis, hepatitis, or other causes, can cripple this function. When the liver can’t store or release glucose effectively, blood sugar becomes much harder to maintain between meals.
Kidney failure is another major predisposing factor. The kidneys play a role in clearing insulin and other drugs from the body, and when they fail, insulin stays active in the bloodstream longer than it should. Kidney disease also disrupts glucose and insulin metabolism through multiple pathways. In people with renal failure, hypoglycemia is often triggered by a combination of factors: insulin therapy, other medications, coexisting liver disease, or infections.
Rare but Serious Causes
A small number of people develop low blood sugar from an insulinoma, a tumor in the pancreas that produces excess insulin on its own. These tumors are uncommon. In one study of emergency department patients presenting with hypoglycemia, only 0.6% had an insulinoma. Even rarer is insulin autoantibody syndrome, where the body produces antibodies that interfere with normal insulin function, and non-islet cell tumors that secrete insulin-like substances.
These conditions are worth knowing about because they cause repeated, unexplained episodes of low blood sugar in people who don’t have diabetes and aren’t taking any medications that affect glucose. If that pattern sounds familiar, it warrants thorough testing.
How Low Is Too Low
The American Diabetes Association classifies hypoglycemia into three levels. Level 1 is a glucose reading below 70 mg/dL but at or above 54 mg/dL. You might feel shaky, sweaty, or irritable, but you can treat it yourself with fast-acting carbohydrates like juice or glucose tablets. Level 2 is below 54 mg/dL, which carries a higher risk of cognitive impairment and requires prompt treatment. Level 3 is a severe event where your mental or physical functioning is altered enough that you need someone else to help you recover, regardless of the specific glucose number.
Mild episodes are common in people on insulin and aren’t always dangerous, but repeated drops below 54 mg/dL can blunt your body’s warning signals over time, making it harder to recognize future lows before they become severe. This phenomenon, sometimes called hypoglycemia unawareness, is one reason recurrent low blood sugar needs to be addressed rather than brushed off.