Low blood sugar, or hypoglycemia, happens when your blood glucose drops below about 70 mg/dL. The most common cause is diabetes medication, particularly insulin or drugs that stimulate insulin production, but dozens of other factors can trigger it in people with or without diabetes. These range from skipping meals and drinking alcohol to rare conditions like pancreatic tumors or adrenal gland disorders.
Diabetes Medications
Insulin is the single most frequent cause of low blood sugar. When you take more insulin than your body needs for the amount of food you’ve eaten or the activity you’ve done, blood glucose drops quickly. This can happen because of a dosing error, a missed or delayed meal after taking insulin, or a change in routine that throws off the balance between medication and food intake.
A class of oral diabetes drugs called sulfonylureas works by pushing the pancreas to release more insulin regardless of what your blood sugar is doing at that moment. That makes them another common culprit. The risk increases if your kidney function has declined over time, because your body clears these drugs more slowly, letting them build up and keep driving insulin secretion longer than intended.
Skipping Meals and Prolonged Fasting
Your liver stores glucose in a compact form called glycogen and releases it between meals to keep blood sugar stable. After about two to six hours without eating, those glycogen stores start to run low. The liver then switches to manufacturing glucose from scratch, using building blocks like amino acids and other small molecules. If you fast for an extended period, especially without much stored glycogen to begin with (from poor nutrition or chronic illness), your blood sugar can dip below normal even without any medication involved.
Alcohol and Low Blood Sugar
Heavy drinking is one of the more underappreciated causes of hypoglycemia. When your liver processes alcohol, it uses up a key molecule (NAD) that it also needs to manufacture new glucose. The more alcohol you drink, the more that molecule gets tied up in alcohol metabolism, and the less glucose your liver can produce. This is especially dangerous if you drink on an empty stomach or after a period of not eating, because your glycogen reserves are already low and your liver’s backup system for making glucose is now impaired. People with alcoholic liver disease face a compounded risk: malnutrition leaves them with less stored glycogen, and liver damage reduces the organ’s overall capacity to regulate blood sugar.
Exercise and Physical Activity
Working muscles consume glucose at a dramatically higher rate than resting muscles. During intense or prolonged exercise, your muscles can burn through available blood glucose and deplete your liver’s glycogen stores faster than your body can replenish them. For people on insulin or sulfonylureas, the risk is even greater because exercise also makes cells more sensitive to insulin, accelerating glucose uptake.
What surprises many people is that low blood sugar can strike hours after a workout, not just during it. This delayed drop happens because your muscles continue pulling in glucose to rebuild their glycogen stores, and your cells remain more insulin-sensitive for up to 24 hours post-exercise. If you take diabetes medication and don’t adjust your dose or eat enough after a hard workout, a blood sugar crash overnight or the next morning is a real possibility.
Reactive Hypoglycemia After Meals
Some people experience low blood sugar two to four hours after eating, a pattern called reactive hypoglycemia. In many cases, the exact cause is never identified. The working theory is that the body overshoots its insulin response to a meal, producing more than necessary and driving blood sugar below normal after the food has been absorbed.
One well-established cause is gastric bypass or other bariatric surgery. When the stomach has been surgically altered, food can move into the small intestine unusually fast, triggering a rapid spike in blood sugar followed by an exaggerated insulin release. The result is a sharp crash. Inherited metabolic disorders and, rarely, certain tumors can also cause this pattern.
Liver and Kidney Disease
The liver is your body’s central glucose factory. It stores glycogen, breaks it down between meals, and manufactures new glucose when stores run low. Severe liver damage from cirrhosis, hepatitis, or other conditions impairs all three of these functions. The liver also clears diabetes medications from the bloodstream, so when it’s not working well, those drugs linger longer and have a stronger effect than intended.
The kidneys serve as a backup glucose producer during prolonged fasting and also help clear insulin and other medications. Advanced kidney disease reduces both of these capacities, making hypoglycemia more likely, particularly in people who take diabetes drugs. The combination of kidney failure and a sulfonylurea is a well-known recipe for dangerous blood sugar drops.
Hormonal Disorders
Several hormones work to raise blood sugar when it starts to fall. Cortisol, produced by the adrenal glands, is one of the most important. It helps the body convert stored nutrients into glucose, especially during illness, injury, or stress. In Addison’s disease (adrenal insufficiency), the adrenal glands don’t produce enough cortisol. Under normal conditions, the adrenal glands ramp up cortisol production to two or three times the usual amount during physical stress. Without that surge, blood sugar can plummet, sometimes severely enough to cause an adrenal crisis with dangerously low blood pressure and glucose levels.
Pituitary gland disorders can have a similar effect, since the pituitary controls the adrenal glands and also produces growth hormone, another player in blood sugar regulation. Children are more vulnerable to hypoglycemia from hormonal deficiencies because they have smaller glycogen reserves relative to their body’s energy demands.
Insulin-Producing Tumors
An insulinoma is a rare tumor in the pancreas that produces insulin continuously, regardless of what your blood sugar is doing. Normally, insulin secretion drops when blood sugar falls. An insulinoma ignores that signal and keeps pumping out insulin, driving glucose lower and lower. Symptoms tend to be worst during fasting or when meals are skipped or delayed, because there’s no incoming food glucose to offset the excess insulin. People with a genetic condition called multiple endocrine neoplasia type 1 are at higher risk. Most insulinomas are small and benign, and surgical removal typically resolves the problem.
Non-Diabetes Medications
A number of drugs prescribed for conditions other than diabetes can lower blood sugar as a side effect. The list includes:
- Beta-blockers used for high blood pressure and heart conditions, which can both lower blood sugar and mask the warning symptoms like a racing heart
- Certain antibiotics, including some fluoroquinolones and the anti-parasitic drug quinine
- Heart rhythm medications like quinidine
- Some pain relievers, particularly indomethacin
The risk is generally higher in people who are already vulnerable, such as those with kidney or liver problems, older adults, or people who aren’t eating well.
How Low Blood Sugar Is Identified
Doctors use a simple three-part checklist called Whipple’s triad to confirm that someone truly has a hypoglycemic disorder rather than symptoms that mimic it. All three criteria must be present: symptoms consistent with low blood sugar (shakiness, sweating, confusion, irritability), a confirmed low glucose reading from a laboratory blood test taken during symptoms, and resolution of those symptoms once blood sugar is brought back up. Meeting all three points distinguishes true hypoglycemia from other conditions that can feel similar, like anxiety or low blood pressure, and guides doctors toward finding the underlying cause.