Low blood glucose, or hypoglycemia, happens when your blood sugar drops below 70 mg/dL (3.9 mmol/L). At that level, your body starts releasing stress hormones to push glucose back up, and you may begin to feel shaky, sweaty, or lightheaded. The causes range from common and easily fixable (a missed meal, too much insulin) to rare conditions that need medical investigation. Here’s what can drive your glucose down and why.
How Your Body Normally Keeps Glucose Steady
Your body treats blood sugar like a thermostat. When levels rise after a meal, your pancreas releases insulin to move glucose into cells. When levels start to fall, your liver steps in by breaking down stored glycogen into glucose and releasing it into your bloodstream. If glycogen runs low, your liver can also build new glucose from scratch using amino acids, lactate, and other raw materials, a process called gluconeogenesis.
Hormones like cortisol and epinephrine act as backup systems, signaling the liver to release more glucose and reducing how much your muscles absorb. Low blood sugar happens when one or more of these systems fails, gets overwhelmed, or is artificially overridden by medication or another factor.
Diabetes Medications
The most common cause of low glucose is diabetes treatment itself. Insulin injected from the outside doesn’t respond to your blood sugar the way your own pancreas would. It keeps working whether your glucose is high, normal, or already falling. If you take too much, eat less than planned, or exercise unexpectedly, the mismatch between insulin and available glucose can send your levels plummeting.
A class of oral medications called sulfonylureas works by stimulating the beta cells of your pancreas to release more insulin into your bloodstream. Unlike some newer diabetes drugs, sulfonylureas don’t adjust their effect based on your current glucose level. They push insulin out regardless, which is why they carry a meaningful risk of lows, especially in older adults or people with irregular eating patterns. Physical exercise makes this worse: in studies of the sulfonylurea glyburide, combining the drug with exercise caused blood sugar to bottom out roughly 30 minutes sooner than taking the drug alone, because exercise didn’t blunt the extra insulin the drug was forcing out.
Skipping Meals and Not Eating Enough
Your liver stores only about 12 to 18 hours’ worth of glycogen. If you skip meals, fast for an extended period, or eat very little over the course of a day, those reserves start to deplete. Once glycogen is low, your body relies entirely on gluconeogenesis, which is slower and produces less glucose per minute. For most healthy people, the body compensates well enough to prevent symptoms. But if you’re also exercising, drinking alcohol, or taking medication that lowers blood sugar, the combination can tip you into hypoglycemia.
Alcohol
Alcohol is one of the more underappreciated causes of low blood sugar. When your liver processes alcohol, it becomes chemically unable to produce new glucose at its normal rate. Research measuring this effect directly found that gluconeogenesis dropped by 45% in the five hours after alcohol consumption, and the availability of the raw materials the liver uses to build glucose fell by 61%. Importantly, alcohol doesn’t stop the liver from breaking down glycogen, so the risk is highest when glycogen stores are already low, such as after a day of poor eating or prolonged fasting. This is why hypoglycemia from alcohol tends to show up after several days of heavy drinking with little food, though it can also happen after a single night of drinking on an empty stomach.
Reactive Hypoglycemia After Meals
Some people experience low blood sugar within four hours of eating, not between meals. This is called reactive or postprandial hypoglycemia. The pattern typically involves a rapid spike in glucose after a meal, followed by an oversized insulin response that drives levels too low.
In people without diabetes, the exact cause often isn’t clear. Known triggers include alcohol and inherited metabolic conditions. One well-documented cause is gastric bypass or other bariatric surgery, which allows food to move into the small intestine much faster than normal. This rapid delivery triggers a surge of insulin that overshoots what’s needed. For many people with reactive hypoglycemia, though, no specific underlying cause is ever identified.
Exercise and Delayed Lows
Physical activity burns through glucose quickly. Your muscles pull sugar from the bloodstream during exercise, and afterward, your body works to replenish the glycogen it burned, continuing to absorb glucose for hours. This is why blood sugar can drop not just during a workout but well after you’ve stopped, sometimes overnight following an evening gym session.
For people without diabetes, the body usually compensates by dialing back insulin and ramping up liver output. But if you’re on insulin or sulfonylureas, those automatic adjustments can’t fully kick in because the medication keeps working independently. The combination of depleted glycogen and ongoing medication is a common setup for unexpected lows.
Adrenal Insufficiency and Cortisol Deficiency
Your adrenal glands produce cortisol, which plays a direct role in keeping blood sugar stable between meals. Cortisol signals the liver to produce glucose and helps limit how much glucose your tissues absorb. When the adrenal glands don’t produce enough cortisol, a condition called adrenal insufficiency, this safety net weakens. The adrenal glands also become less able to produce epinephrine, the hormone that triggers emergency glucose release. People with adrenal insufficiency lose both their steady-state glucose support and their emergency backup, making them vulnerable to lows during stress, illness, or fasting.
Liver and Kidney Disease
Since the liver is the primary organ responsible for releasing glucose between meals, severe liver disease can directly impair its ability to keep up. A damaged liver may not store glycogen efficiently or produce new glucose fast enough to meet demand.
The kidneys also contribute to blood sugar regulation in ways many people don’t realize. They produce a small but meaningful amount of glucose on their own and help clear insulin from the bloodstream. Chronic kidney disease can impair both of these functions: less glucose is produced, and insulin lingers longer than it should. For people with diabetes and kidney disease, this means the same dose of medication may suddenly become too strong as kidney function declines.
Insulin-Producing Tumors
Rarely, a tumor on the pancreas called an insulinoma produces insulin continuously, regardless of blood sugar levels. This leads to repeated episodes of hypoglycemia, often with glucose dropping below 40 mg/dL. The hallmark is the combination of low blood sugar, symptoms of hypoglycemia, and relief of those symptoms once glucose is restored. About 95% of insulinomas can be identified through a supervised fasting test lasting 48 hours, during which doctors track glucose, insulin, and related markers. In roughly 5% of cases, fasting doesn’t provoke a low, but eating a meal does, so testing after a standardized meal can catch the rest.
How Severity Is Classified
The American Diabetes Association’s 2025 guidelines break hypoglycemia into three levels. Level 1 is a glucose between 54 and 69 mg/dL, the range where you’ll likely feel early warning signs like shakiness, hunger, or a fast heartbeat. Level 2 is below 54 mg/dL, where symptoms become more serious and cognitive function starts to decline. Level 3 is any episode severe enough that you need someone else’s help to recover, regardless of the specific number on the meter. This could mean confusion, seizures, or loss of consciousness.
What to Do When Glucose Drops
The standard approach is straightforward: eat 15 grams of fast-acting carbohydrates, wait 15 minutes, then recheck your blood sugar. If it’s still at or below 70 mg/dL, repeat. Pure glucose tablets or glucose gel work fastest. Juice or regular soda also works, but avoid reaching for chocolate, peanut butter crackers, or other foods with fat or protein as your first choice. Fat slows down how quickly glucose reaches your bloodstream, and protein can actually trigger more insulin release, which is the opposite of what you need in the moment.
If you use an automated insulin delivery system (a closed-loop insulin pump), the carbohydrate dose is typically smaller, around 5 to 10 grams, because the pump is already reducing insulin delivery on its own. The exception is if the low happened during exercise or after a large meal bolus, in which case the full 15 grams is appropriate.