Blood sugar drops when your body uses or stores glucose faster than it can replace it. This can happen from medications, missed meals, exercise, alcohol, or less common medical conditions. A blood sugar level below 70 mg/dL is considered low, and below 54 mg/dL is the point where brain function starts to suffer and you need to act fast.
How Your Body Regulates Blood Sugar
Understanding why blood sugar drops starts with understanding what keeps it steady. After you eat, carbohydrates break down into glucose, which enters your bloodstream. Your pancreas responds by releasing insulin, which acts like a key that unlocks your cells so glucose can move inside for energy. Specifically, insulin triggers special glucose transporters to travel from deep inside your muscle and fat cells to the cell surface. Up to 50% of these transporters relocate to the outer membrane when insulin signals them, dramatically increasing the rate at which cells pull glucose out of the blood.
When this system works well, blood sugar rises gently after a meal and then comes back down as cells absorb the glucose. Between meals, your liver steps in by releasing stored glucose (glycogen) and by manufacturing new glucose from raw materials like amino acids and lactate, a process called gluconeogenesis. A blood sugar drop happens when one side of this equation gets thrown off: too much glucose leaving the blood, not enough glucose entering it, or both.
Diabetes Medications That Push Glucose Too Low
The most common cause of significant blood sugar drops is diabetes medication, particularly insulin and a class of oral drugs called sulfonylureas. Sulfonylureas work by binding to receptors on the insulin-producing cells of the pancreas, forcing them to release preformed insulin regardless of whether blood sugar is actually high. This “always on” mechanism means that if you take the medication but eat less than expected, exercise more, or miscalculate timing, blood sugar can fall well below normal.
Not all diabetes drugs carry this risk equally. Among sulfonylureas, glyburide causes more hypoglycemia than glipizide or glimepiride, especially in older adults and people with kidney problems. Meanwhile, many newer diabetes medications, including metformin, don’t typically push blood sugar below normal even in overdose situations. These drugs work by reducing excess glucose rather than forcing insulin release, which is why they’re technically called “antihyperglycemic” rather than “hypoglycemic” agents. If you take insulin, the risk depends on the type, dose, and timing relative to meals and activity.
Skipping Meals and Reactive Drops After Eating
Missing a meal is one of the simplest ways blood sugar falls. Your liver can cover the gap for a while by releasing stored glycogen, but that supply is limited. If you go long enough without eating, especially if you’re also on medication or have been physically active, your body can’t keep up.
Paradoxically, eating can also trigger a drop. Reactive hypoglycemia happens when blood sugar spikes rapidly after a meal, prompting your body to overshoot with insulin, sending glucose crashing 2 to 4 hours later. Foods that cause the fastest spikes are the biggest culprits: soda, candy, popsicles, and other pure-sugar items with no fat or protein to slow absorption. The more carbohydrates in a meal, the higher the initial spike and the steeper the potential fall. Pairing carbs with protein or fat slows the whole cycle down and reduces the chance of a reactive drop.
Exercise and Delayed Drops
Physical activity pulls glucose into working muscles, which is why moderate aerobic exercise, like walking, jogging, or cycling, reliably lowers blood sugar, especially when done after a meal while insulin from that meal is still active. Prolonged aerobic activity can cause exaggerated decreases.
High-intensity exercise behaves differently. Brief, very intense efforts like sprinting or heavy weightlifting can actually raise blood sugar temporarily because stress hormones trigger your liver to dump glucose into the bloodstream. Mixed activities, like interval training or team sports that alternate bursts of effort with moderate movement, tend to produce the most stable blood sugar readings. Inserting short high-intensity bursts into a longer moderate workout also slows blood sugar decline.
The tricky part is what happens afterward. Post-exercise blood sugar drops typically occur 6 to 15 hours later, as muscles replenish their glucose stores, though the risk window can extend out to 48 hours. This means a late-afternoon workout could cause a low in the middle of the night.
How Alcohol Blocks Glucose Production
Alcohol interferes with your liver’s ability to make new glucose. In fasted men, gluconeogenesis dropped by 45% in the five hours after drinking, with the availability of raw materials the liver uses to build glucose falling by 61%. Your liver partially compensates by releasing more stored glycogen, which is why a couple of drinks on a full stomach might not cause problems. But if your glycogen stores are already low from skipping meals, dieting, or exercising, alcohol removes the backup plan. The combination of alcohol with any of those other factors raises the risk considerably.
This also helps explain why drinking before bed is a risk factor for nocturnal hypoglycemia, where blood sugar falls below 70 mg/dL during sleep. Other triggers for nighttime drops include skipping dinner, exercising in the evening, and taking certain types of insulin that peak six to eight hours after injection, right in the middle of the night.
Medical Conditions Beyond Diabetes
People without diabetes can also experience significant blood sugar drops. One uncommon but important cause is an insulinoma, a tumor on the pancreas that produces excess insulin. These tumors are usually small and benign, but they force the body to absorb glucose continuously, causing repeated lows. Certain genetic conditions raise the risk, including multiple endocrine neoplasia type 1 and Von Hippel-Lindau syndrome.
Other non-diabetes causes include hormonal deficiencies (particularly cortisol or growth hormone, which normally help raise blood sugar), severe liver disease, prolonged starvation, and critical illness. Gastric bypass surgery can also cause blood sugar drops through a mechanism similar to reactive hypoglycemia, where food enters the small intestine too quickly and triggers an outsized insulin response.
Recognizing the Symptoms
Your body sends two waves of warning signs as blood sugar falls. The first wave comes from your stress response: sweating, shakiness, a fast heartbeat, anxiety, and sudden hunger. These symptoms kick in as your body releases adrenaline to counteract the drop. They’re uncomfortable but useful, because they tell you something is wrong while you can still act easily.
If blood sugar keeps falling, a second set of symptoms appears as the brain itself runs short on fuel: weakness, dizziness, difficulty concentrating, confusion, blurred vision, and behavior that can look like intoxication. At the most severe level, altered mental or physical functioning makes it impossible to treat yourself, and you need someone else’s help. This progression is why catching the early symptoms matters.
What to Do When Blood Sugar Drops
The standard approach is the 15-15 rule: eat or drink 15 grams of fast-acting carbohydrates, then wait 15 minutes and check your blood sugar again. Fifteen grams looks like about 4 ounces of juice, 3 to 4 glucose tablets, or a tablespoon of sugar. If your level is still below 70 mg/dL, repeat. Once it’s back up, eat a small meal or snack with protein and complex carbs to keep it stable.
For preventing drops in the first place, the common thread across nearly every cause is a mismatch between glucose supply and glucose demand. Eating regular meals, adjusting medication timing around exercise, limiting alcohol on an empty stomach, and pairing carbohydrates with protein or fat all work by keeping that balance closer to center.