Low blood glucose, or hypoglycemia, happens when your blood sugar drops below 70 mg/dL. The most common cause is diabetes medication, particularly insulin, but dozens of other triggers exist in people with and without diabetes. Understanding the specific cause matters because the fix is different for each one.
How Your Body Normally Prevents Low Blood Sugar
Your body runs a tightly controlled system to keep blood sugar in a safe range. When levels start to fall, your pancreas releases a hormone called glucagon, which signals the liver to convert its stored sugar (glycogen) into glucose and push it into the bloodstream. The liver can also build new glucose from scratch using proteins and other raw materials, a process called gluconeogenesis. These two backup systems are especially important during fasting and sleep, since your brain depends on a steady glucose supply to function.
Hypoglycemia happens when something disrupts this safety net. Either too much insulin is pulling sugar out of your blood, your liver can’t release enough glucose to keep up, or both.
Diabetes Medication: The Most Common Cause
If you have diabetes, the most frequent trigger is a mismatch between your medication and your body’s actual need for it. The main scenarios include:
- Taking too much insulin or other glucose-lowering medication. Even a small dosing error can push blood sugar below safe levels.
- Eating less than usual after taking your normal dose. Your medication was calibrated for a certain amount of food, and without it, blood sugar drops.
- Skipping or delaying a meal or snack. The medication keeps working whether you eat or not.
- Exercising more than usual without adjusting food intake or medication. Physical activity pulls glucose into muscles, compounding the effect of insulin.
These triggers often overlap. A person who takes their usual insulin, eats a smaller lunch, and then goes for a long walk is stacking three risk factors at once.
Medications That Lower Blood Sugar as a Side Effect
Insulin gets the most attention, but several other drug classes can cause hypoglycemia. Sulfonylureas (glipizide, glimepiride, glyburide) are among the most common culprits because they stimulate the pancreas to release more insulin regardless of what your blood sugar is actually doing. When metformin is combined with a sulfonylurea, the risk increases further.
Beyond diabetes drugs, some less obvious medications carry hypoglycemia risk. Certain antibiotics used for infections, including some fluoroquinolones and the antimalarial drug quinine, can lower blood sugar. Some heart rhythm medications, certain pain relievers, and beta-blockers (which can also mask the warning symptoms of a low) round out the list. Beta-blockers are particularly tricky because they blunt the rapid heartbeat and shakiness that would normally alert you to a drop.
Even accidentally taking someone else’s diabetes medication is a recognized cause, especially in households where multiple people use different prescriptions.
Alcohol and the Liver
Drinking alcohol, especially on an empty stomach, directly interferes with your liver’s ability to produce glucose. In one study, alcohol reduced the liver’s glucose-building capacity by 45% over five hours compared to a placebo. The availability of raw materials the liver needs for glucose production dropped by roughly 61%.
The liver partially compensates by releasing more of its stored glycogen, which is why blood sugar may not plummet immediately. But if glycogen stores are already low from fasting or skipping meals, the safety net disappears. This is why alcohol can cause delayed hypoglycemia hours after your last drink, catching people off guard the next morning. Heavy drinking without eating is the highest-risk scenario.
Reactive Hypoglycemia: The Post-Meal Drop
Some people experience low blood sugar not from fasting but after eating, typically within four hours of a meal. This is called reactive or postprandial hypoglycemia. It happens when your body overreacts to a rise in blood sugar by releasing too much insulin, which then drives glucose levels below where they started.
Meals high in sugar or processed simple carbohydrates, like white bread or sugary snacks eaten on an empty stomach, are the most common triggers. The rapid spike in blood sugar provokes an outsized insulin response, and the crash follows. In many cases, the exact mechanism isn’t fully understood, but shifting toward meals that combine protein, fat, and complex carbohydrates tends to smooth out the blood sugar curve and prevent the overshoot.
Hypoglycemia After Bariatric Surgery
People who have had gastric bypass surgery face a specific form of post-meal hypoglycemia. The rearranged anatomy causes food to empty from the small stomach pouch directly into the intestine much faster than normal. This rapid delivery triggers an earlier and higher spike in blood sugar, which in turn provokes an exaggerated release of insulin driven by gut hormones called incretins.
The result is a sharp rise followed by a steep drop, often within one to two hours of eating. Several other factors pile on: the body’s glucagon response to low blood sugar becomes weaker after surgery, insulin sensitivity increases, and insulin is cleared from the bloodstream more slowly. Together, these changes make post-surgical hypoglycemia a persistent challenge for some patients, requiring careful attention to meal composition and portion size.
Organ Disease and Critical Illness
Several serious medical conditions can cause hypoglycemia by impairing the body’s glucose production or clearance systems. The liver is responsible for most of your body’s glucose output, so severe liver diseases like advanced cirrhosis or severe hepatitis can directly reduce the supply. When the liver is badly damaged, it simply cannot store or release enough glycogen to maintain blood sugar during fasting.
Kidney disease contributes in a different way. The kidneys help clear medications from your body, and when they fail, drugs that lower blood sugar can build up to dangerous levels. This makes kidney disease a compounding risk factor on top of whatever other medications a person takes. Advanced heart disease and severe infections also increase glucose demand while impairing the body’s ability to produce it, creating a dangerous gap.
Insulinomas and Hormone Deficiencies
In rare cases, a small tumor on the pancreas called an insulinoma produces excess insulin on its own, flooding the body with the hormone. People with insulinomas typically experience low blood sugar during fasting, most often first thing in the morning. Doctors suspect it when a patient shows three things together: symptoms of low blood sugar, a confirmed glucose reading below 55 mg/dL, and improvement after eating carbohydrates. Most insulinomas are benign and can be surgically removed.
Deficiencies in other hormones can also contribute. Cortisol and growth hormone both play roles in maintaining blood sugar during fasting. When the adrenal glands or pituitary gland underperform, one of the consequences can be recurrent hypoglycemia, particularly during periods of stress or illness when glucose demands rise.
How to Respond to a Low Blood Sugar Episode
The standard approach is the 15-15 rule: eat 15 grams of fast-acting carbohydrates, wait 15 minutes, then check your blood sugar again. If it’s still below 70 mg/dL, repeat the process. Good sources of 15 grams of carbs include four glucose tablets, four ounces of juice, or a tablespoon of honey. Once blood sugar returns to normal, eating a balanced snack or meal helps prevent another drop.
If episodes keep happening without an obvious explanation like medication timing or missed meals, that pattern itself is worth investigating. Recurrent hypoglycemia in someone without diabetes can signal an underlying condition, from a hormone deficiency to a rare tumor, that requires a different kind of treatment entirely.