Blood sugar drops when your body has more insulin than it needs relative to the glucose available in your bloodstream. The most common cause is diabetes medication, particularly insulin, but skipping meals, drinking alcohol, intense exercise, and certain medical conditions can all push blood sugar below the normal threshold of 70 mg/dL.
How Your Body Regulates Blood Sugar
Your blood sugar stays stable through a balancing act between two hormones. Insulin drives glucose out of your bloodstream and into muscle, fat, and liver cells for storage. Glucagon does the opposite: it signals your liver to break down its stored glucose and release it back into the blood when levels fall too low. When this system works properly, your body catches a dip before you ever feel it. Blood sugar drops happen when something disrupts this balance, either by flooding the system with too much insulin, depleting your glucose stores, or blocking your liver’s ability to produce new glucose.
Diabetes Medications
The single most common reason for low blood sugar is a side effect of medications used to treat diabetes. If you take insulin or certain oral medications, your blood sugar can drop too low in three situations: you took your usual dose but ate less than normal, you exercised more than usual without adjusting your medication, or you simply took too much. Sulfonylureas (like glipizide and glyburide) are particularly well-known for triggering lows because they stimulate your pancreas to release insulin regardless of what your blood sugar is doing at the time.
Metformin on its own rarely causes low blood sugar, but when combined with a sulfonylurea, the risk increases significantly. The same applies to several other diabetes drug classes, including SGLT2 inhibitors and thiazolidinediones, which can drive blood sugar down when paired with sulfonylureas.
Skipping or Delaying Meals
Your liver stores enough glucose to keep you going for several hours between meals, but those reserves are limited. If you skip a meal, eat significantly less than usual, or go long stretches without food, your blood sugar can fall below normal, especially if you’re also taking medication that lowers glucose. Even without diabetes, some people are more sensitive to gaps between meals. The body’s backup system, glucagon signaling the liver to release stored glucose, works well in healthy individuals but can be sluggish or impaired in people with diabetes or certain hormonal conditions.
Alcohol
Alcohol is one of the most underestimated causes of low blood sugar. When you drink, your liver prioritizes breaking down alcohol over its other jobs, including producing new glucose. Specifically, alcohol changes the chemical environment inside liver cells in a way that blocks the conversion of other nutrients into glucose. This means your liver essentially stops topping off your blood sugar while it processes the alcohol. The effect is strongest when you drink on an empty stomach or after prolonged fasting, and it can cause blood sugar to drop hours after your last drink, sometimes even the next morning.
Exercise and Physical Activity
During exercise, your muscles pull glucose from your blood at a much higher rate than normal. Physical activity triggers your muscle cells to move glucose transporters to their surface, essentially opening more doors for glucose to enter. This process doesn’t require insulin, which is why exercise lowers blood sugar even in people who are insulin resistant.
What catches many people off guard is that the effect doesn’t stop when the workout ends. In the first two hours after exercise, your muscles continue absorbing glucose at an elevated rate. And your body’s sensitivity to insulin remains heightened for up to 48 hours after a single bout of endurance exercise. This means blood sugar can drop well after you’ve finished exercising, including overnight if you worked out in the evening. For people on insulin or sulfonylureas, this delayed effect is a common cause of unexpected lows.
Reactive Hypoglycemia After Meals
Some people experience blood sugar drops within four hours after eating, a pattern called reactive hypoglycemia. In people without diabetes, the exact cause is often unclear, but it likely involves the pancreas releasing more insulin than the meal actually required. The blood sugar rises normally after eating, but then overshoots on the way back down.
People who have had gastric bypass or other bariatric surgery are especially prone to this. After these procedures, food moves into the small intestine faster than normal, triggering a rapid and exaggerated insulin response. Alcohol, inherited metabolic conditions, and rarely, certain tumors can also cause post-meal drops.
Non-Diabetes Medications
Several medications unrelated to diabetes can lower blood sugar as a side effect. These include:
- Beta-blockers (used for high blood pressure and heart conditions), which can both lower blood sugar and mask the warning symptoms like rapid heartbeat
- Certain antibiotics and anti-infection drugs, including some fluoroquinolones, quinine, and trimethoprim-sulfamethoxazole
- Heart rhythm medications like quinidine
- Some pain relievers like indomethacin
People with kidney failure are at higher risk from these medications because their kidneys clear the drugs more slowly, intensifying the blood-sugar-lowering effect. Accidentally taking someone else’s diabetes medication is another well-documented cause, particularly in households where multiple people take different prescriptions.
Rare Medical Conditions
When blood sugar drops repeatedly without an obvious explanation like medication, diet, or exercise, a few uncommon conditions are worth investigating. An insulinoma is a rare tumor in the pancreas that continuously releases insulin, flooding the body regardless of whether glucose levels are high or low. About 85% to 90% of these tumors stay contained within the pancreas and don’t spread. Surgery is the primary treatment, and the 10-year survival rate after surgical removal is 87%.
Adrenal insufficiency, where the adrenal glands don’t produce enough cortisol, can also lead to low blood sugar. Cortisol normally helps maintain glucose levels between meals, so without enough of it, blood sugar regulation suffers. Pituitary gland disorders can have a similar effect by disrupting the hormones that control cortisol and growth hormone production.
Recognizing the Symptoms
Your body sends two waves of warning signals as blood sugar falls. The first wave comes from your nervous system reacting to the drop: sweating, a pounding or racing heart, trembling hands, anxiety, and sudden intense hunger. These symptoms typically appear when blood sugar falls below 70 mg/dL and serve as an early alarm.
If blood sugar continues falling below about 54 mg/dL, a second set of symptoms emerges as the brain itself runs short on fuel. These include confusion, difficulty concentrating, irritability, slurred speech, and in severe cases, loss of consciousness. The early nervous-system symptoms usually come first, giving you time to act. But people who experience frequent lows can lose these early warning signs over time, a dangerous condition called hypoglycemia unawareness.
What to Do When Blood Sugar Drops
The standard approach is the 15-15 rule recommended by the CDC: eat or drink 15 grams of fast-acting carbohydrates (about four glucose tablets, half a cup of juice, or a tablespoon of sugar), then wait 15 minutes and check your blood sugar again. If it’s still below 70 mg/dL, repeat. Keep repeating until your levels return to your target range. Young children, especially infants and toddlers, typically need less than 15 grams per round.
Severe episodes, where someone is confused, unconscious, or unable to swallow safely, require help from another person. This is classified as a level 3 hypoglycemic event and may need emergency treatment with injectable glucagon, which signals the liver to dump its glucose stores rapidly into the bloodstream.