Blood sugar can drop below normal levels for a wide range of reasons, from diabetes medications and skipped meals to alcohol, intense exercise, and underlying health conditions. A blood sugar reading below 70 mg/dL is considered low, and anything below 54 mg/dL is serious enough to impair your ability to think clearly or function normally.
Some causes are common and easy to fix. Others point to something deeper going on in your body. Here’s what can trigger a drop and why it happens.
Diabetes Medications
The most common cause of low blood sugar is medication used to treat diabetes. Insulin, whether injected or delivered by a pump, directly lowers glucose in your bloodstream. If the dose is slightly too high, or if you take it and then eat less than expected or skip a meal entirely, your blood sugar can fall fast.
A class of oral medications called sulfonylureas carries the same risk. These drugs work by stimulating your pancreas to release more insulin, and they do so regardless of how much sugar is actually in your blood at that moment. Taking more than prescribed, or combining them with other blood sugar-lowering drugs, increases the chance of a drop. The effect can also be unpredictable if your eating schedule changes or your kidney function slows down, since your body may not clear the medication as quickly.
Skipping Meals or Not Eating Enough
Your body constantly pulls glucose from your bloodstream to fuel your brain, muscles, and organs. When you go several hours without eating, your liver is supposed to release stored glucose to keep levels steady. But this backup system has limits. If your glycogen stores are low (from dieting, fasting, or heavy exercise the day before), your blood sugar can dip below normal.
This is especially relevant if you’re on any blood sugar-lowering medication, but it can happen to people without diabetes too, particularly children and older adults whose reserves are smaller.
Alcohol
Drinking alcohol, especially on an empty stomach, is one of the more underappreciated causes of low blood sugar. Your liver is responsible for producing new glucose between meals through a process called gluconeogenesis. Alcohol directly interferes with this process. Research published in the American Journal of Physiology found that alcohol reduced the liver’s ability to produce glucose by roughly 61% compared to baseline.
This means your liver essentially stops topping off your blood sugar while it’s busy processing alcohol. The drop can happen hours after your last drink, sometimes even the next morning, which is why some people wake up shaky or lightheaded after a night of heavy drinking.
Exercise
Physical activity pulls glucose out of your bloodstream and into your muscles for energy. That’s a good thing for blood sugar management overall, but it can sometimes overshoot. According to the American Diabetes Association, a single workout can lower your blood sugar for up to 24 hours afterward by making your cells more responsive to insulin.
This delayed effect catches people off guard. You might feel fine during a run or a gym session and then experience a drop later that evening or overnight. The risk is higher with prolonged or intense exercise, and it increases significantly if you haven’t eaten enough beforehand or if you’re taking insulin or sulfonylureas.
Reactive Hypoglycemia After Meals
Some people experience blood sugar drops within four hours of eating, a pattern known as reactive hypoglycemia. It sounds counterintuitive since eating raises blood sugar, but in some cases the body overreacts with too much insulin, sending levels crashing down after they initially spike.
In people without diabetes, the exact cause often isn’t clear, but it tends to be linked to what and when you eat. Meals high in refined carbohydrates can trigger a sharp glucose spike followed by an exaggerated insulin response. Eating smaller, balanced meals with protein, fat, and fiber tends to smooth out this pattern.
Gastric Bypass and Bariatric Surgery
People who have had gastric bypass surgery are at particular risk for post-meal blood sugar drops, sometimes severe ones. The surgery changes how quickly food reaches the small intestine, which triggers an exaggerated release of gut hormones that stimulate insulin production. Research in the Cleveland Clinic Journal of Medicine found that post-bypass patients released significantly more insulin after eating glucose compared to people who hadn’t had the surgery, with an even more dramatic response in those experiencing repeated low blood sugar episodes.
Over time, this hormonal overstimulation can cause the insulin-producing cells in the pancreas to physically enlarge and multiply, making the problem worse. Some patients don’t develop symptoms until months or even years after surgery, which makes it easy to miss the connection.
Liver and Kidney Disease
Your liver stores glucose and releases it between meals. When the liver is severely damaged by conditions like cirrhosis or advanced hepatitis, it loses the ability to perform this role effectively. The result is blood sugar that drops during fasting or overnight.
Kidney disease creates a different problem. Your kidneys help clear medications from your body. When they aren’t working well, blood sugar-lowering drugs can build up in your system and keep working longer and harder than intended, pushing glucose levels dangerously low. Even medications not designed to lower blood sugar can accumulate and have unexpected effects on glucose when kidney function declines.
Adrenal Insufficiency
Cortisol, produced by your adrenal glands, plays a direct role in helping your body convert food into energy and keeping blood sugar stable between meals. In conditions like Addison’s disease, the adrenal glands don’t produce enough cortisol, and low blood sugar becomes a recurring problem.
Secondary adrenal insufficiency, where the issue originates in the pituitary gland rather than the adrenals themselves, is actually more likely to cause noticeable blood sugar drops. Other symptoms typically include fatigue, muscle weakness, and unintentional weight loss, which together can prompt testing.
Non-Diabetes Medications
Several medications that have nothing to do with diabetes can lower blood sugar as a side effect. These include:
- Beta-blockers used for high blood pressure and heart conditions
- Certain antibiotics, including some fluoroquinolones and a combination antibiotic often prescribed for urinary tract infections
- Quinine, used to treat malaria
- Some heart rhythm medications
- Certain pain relievers, including indomethacin
Beta-blockers deserve extra caution because they can also mask the typical warning signs of low blood sugar, like a racing heart and shakiness, making it harder to recognize a drop before it becomes serious.
Insulin-Producing Tumors
Rarely, a tumor on the pancreas called an insulinoma can cause the body to produce far too much insulin regardless of what you eat. These tumors are almost always benign, but they cause repeated episodes of low blood sugar, particularly during fasting or between meals.
Diagnosis typically involves a supervised fasting test in a hospital setting, where doctors monitor blood sugar and insulin levels over time. In a healthy person, insulin levels drop as blood sugar falls. In someone with an insulinoma, insulin stays abnormally high even as glucose drops below 40 mg/dL. The ratio between insulin and glucose rises in a pattern that healthy individuals don’t show. Surgical removal of the tumor is usually curative.
What a Blood Sugar Drop Feels Like
Mild low blood sugar (between 54 and 70 mg/dL) typically causes shakiness, sweating, hunger, irritability, and a fast heartbeat. These are your body’s early warning signals driven by adrenaline. Below 54 mg/dL, symptoms shift toward the brain: confusion, difficulty speaking, blurred vision, and poor coordination. At the most severe level, you may need someone else’s help to recover, and loss of consciousness or seizures are possible.
If you can check your blood sugar and it’s low, the standard approach is the 15-15 rule recommended by the CDC: eat 15 grams of fast-acting carbohydrates (four glucose tablets, four ounces of juice, or a tablespoon of honey), wait 15 minutes, and recheck. Repeat if your levels haven’t come back up. If episodes keep happening without an obvious explanation like a missed meal or too much medication, that pattern is worth investigating.