Low blood sugar, or hypoglycemia, happens when your blood glucose drops below 70 mg/dL. The most common cause is diabetes medication, particularly insulin and a class of drugs called sulfonylureas. But low blood sugar can also affect people without diabetes, triggered by alcohol, skipped meals, liver problems, hormone deficiencies, or rare tumors in the pancreas.
How Your Body Normally Keeps Blood Sugar Stable
Your liver is the central player in blood sugar regulation. It stores glucose in a compact form called glycogen and releases it into the bloodstream whenever levels start to dip. When you haven’t eaten for several hours, the liver breaks down those glycogen stores to keep your brain and blood cells fueled. After an overnight fast, glycogen is largely depleted, so the liver switches to manufacturing new glucose from other raw materials like amino acids and lactate.
This system relies on a careful hormonal balance. When blood sugar falls, your pancreas releases glucagon, which signals the liver to push glucose out. Your brain also detects the drop and triggers the release of stress hormones like adrenaline, which is why low blood sugar often feels like anxiety or a surge of nervous energy. Problems at any point in this chain, whether it’s the liver, the pancreas, the hormones, or the brain’s sensing ability, can lead to hypoglycemia.
Diabetes Medications
If you have diabetes, medication is by far the most likely reason for a low blood sugar episode. Insulin is the biggest culprit because it directly lowers blood glucose, and taking too much, mistiming a dose, or exercising more than expected can tip you into hypoglycemia. Sulfonylureas (such as glipizide, glimepiride, and glyburide) stimulate the pancreas to produce more insulin regardless of what your blood sugar is doing, which makes lows common with these drugs as well.
Several other diabetes medications can contribute, especially in combination with sulfonylureas. These include glinides, certain SGLT2 inhibitors, thiazolidinediones, and even metformin when paired with a sulfonylurea. Common triggers on top of the medication itself include skipping or delaying meals, eating less than usual, exercising harder or longer than planned, and drinking alcohol.
Alcohol
Alcohol is one of the most common non-drug causes of low blood sugar. It blocks the liver’s ability to both break down glycogen and manufacture new glucose, essentially shutting down the two main rescue systems your body relies on. People who drink heavily often have low glycogen reserves to begin with due to poor nutrition, which compounds the problem. Alcohol also suppresses the shivering reflex, weakening one of the body’s natural responses to hypoglycemia. This is why dangerously low blood sugar can develop during or after a night of heavy drinking, sometimes hours after the last drink.
Skipping Meals and Prolonged Fasting
Your liver can typically maintain blood sugar through an overnight fast without trouble. But if you go significantly longer without eating, especially while physically active, glycogen stores run out and your liver’s ability to produce new glucose may not keep up with demand. This is more likely to cause problems if you’re also taking medications that lower blood sugar or if you have an underlying condition affecting the liver or hormones.
Some people experience what’s called reactive hypoglycemia, where blood sugar drops two to four hours after eating rather than during fasting. This happens when the body overshoots its insulin response to a meal, particularly meals high in refined carbohydrates. It’s also relatively common after weight-loss surgery (bariatric procedures), because food moves through the digestive system faster, triggering a rapid and exaggerated insulin spike.
Liver and Kidney Disease
Since the liver is responsible for storing glycogen and producing glucose on demand, liver failure directly undermines both of those functions. People with advanced liver disease have diminished glycogen stores and impaired ability to synthesize new glucose, making them vulnerable to drops.
Kidney disease causes hypoglycemia through a different pathway. The kidneys actually contribute some glucose production on their own, so losing that function removes a backup source. Kidney failure also slows the clearance of insulin and diabetes medications from the body, meaning their blood-sugar-lowering effects last longer than intended. Combined with the poor appetite and malnutrition that often accompany kidney disease, this creates a real risk of lows.
Hormone Deficiencies
Several hormones help raise blood sugar when it drops, and deficiencies in any of them can leave your body unable to mount a proper response. Adrenal insufficiency (where the adrenal glands don’t produce enough cortisol) leads to decreased glucose production and increased sensitivity to insulin, a combination that makes hypoglycemia more likely. Pituitary gland failure can have a similar effect by reducing the hormones that normally stimulate the adrenals and promote growth hormone release.
These conditions are uncommon, but they’re worth investigating when someone without diabetes has repeated episodes of unexplained low blood sugar, especially during fasting or illness.
Severe Illness and Infection
Critical illness, including severe infections and sepsis, can drive blood sugar dangerously low. The mechanism is twofold: the body’s glucose stores are depleted faster because of the intense metabolic demands of fighting infection, and the liver’s ability to produce new glucose is impaired at the same time. This type of hypoglycemia typically occurs in hospitalized patients and is a sign of how seriously the body is under stress.
Insulinomas and Other Rare Causes
An insulinoma is a small tumor in the pancreas that produces insulin continuously, regardless of blood sugar levels. These are rare, occurring in roughly 1 to 4 people per million, but they’re an important cause to identify because they’re usually curable with surgery. The hallmark is repeated episodes of low blood sugar, particularly during fasting, that resolve immediately when you eat. Because insulinoma cells secrete a higher proportion of proinsulin (a precursor to insulin) than normal pancreatic cells, elevated proinsulin levels in the blood can help confirm the diagnosis.
Other rare causes include autoimmune conditions where the body produces antibodies against insulin or insulin receptors, certain large non-pancreatic tumors that produce insulin-like growth factors, and inherited metabolic disorders like glycogen storage disease. Eating disorders such as anorexia nervosa can also cause hypoglycemia through severe calorie restriction and depleted glycogen reserves.
How Low Blood Sugar Feels
Early symptoms come from your body’s adrenaline response: shakiness, sweating, a pounding heart, hunger, and anxiety. These warning signs typically appear when blood sugar drops below 70 mg/dL. If levels continue to fall below 54 mg/dL (classified as level 2 hypoglycemia by the American Diabetes Association), the brain starts running short on fuel. That’s when you may feel confused, drowsy, weak, or have difficulty speaking. At the most severe level, you may need someone else’s help to treat the episode, and untreated severe hypoglycemia can lead to seizures or loss of consciousness.
One particularly dangerous phenomenon is hypoglycemia unawareness. In people who experience frequent low blood sugar episodes, the brain gradually adapts to lower glucose levels and resets its alarm threshold downward. The normal adrenaline-driven warning signs (shaking, sweating, rapid heartbeat) become muted or disappear entirely. This means blood sugar can drop to dangerous levels without any noticeable symptoms, increasing the risk of a severe episode. Hypoglycemia unawareness is most common in people with long-standing type 1 diabetes, but it can develop in anyone with recurrent lows. Avoiding hypoglycemia for several weeks can often restore some awareness by allowing the brain’s glucose-sensing threshold to reset back to normal.
How Hypoglycemia Is Confirmed
A single low blood sugar reading doesn’t necessarily mean you have a hypoglycemia disorder. The standard for diagnosis is called Whipple’s triad: you have symptoms of low blood sugar, a measured blood glucose below 55 mg/dL, and those symptoms resolve once your blood sugar is brought back up. All three components need to be present. This distinction matters because many of the symptoms of hypoglycemia (shakiness, anxiety, lightheadedness) overlap with other conditions, and confirming that low glucose is actually the cause prevents unnecessary testing and treatment for something else entirely.