Blood sugar drops when your body produces too much insulin, doesn’t release enough stored glucose, or both. For people with diabetes, the threshold is a reading below 70 mg/dL. But low blood sugar can happen to anyone, and the causes range from medication side effects and skipped meals to alcohol, hormonal conditions, and even past surgeries.
Diabetes Medications Are the Most Common Cause
Insulin and a class of oral medications called sulfonylureas are responsible for the vast majority of low blood sugar episodes. Sulfonylureas work by stimulating your pancreas to push out more insulin, and they do this regardless of whether your blood sugar is high, normal, or already trending low. If you take more than prescribed, skip a meal after dosing, or combine sulfonylureas with insulin, your blood sugar can fall quickly.
Insulin itself carries the same risk. The dose your doctor calculates is based on expected food intake and activity levels. If any of those variables change, you can end up with more insulin circulating than your body needs, and glucose gets pulled out of your blood faster than it can be replaced. This is why blood sugar drops are especially common after unexpected exercise, a smaller meal than usual, or a delayed meal.
How Alcohol Disrupts Blood Sugar
Your liver normally acts as a backup generator for blood sugar. When levels start falling between meals or overnight, it manufactures new glucose and releases it into your bloodstream. Alcohol disrupts that process. When your liver is busy breaking down alcohol, the byproducts block it from producing new glucose. Blood sugar can fall rapidly as a result.
What makes alcohol particularly risky is the timeline. Drinking can affect your blood sugar for up to 12 hours, according to Johns Hopkins Medicine. That means a few drinks in the evening can cause a dangerous low in the middle of the night or the next morning, well after you’ve stopped drinking. This catches people off guard because they don’t connect the timing of the drop to the alcohol consumed hours earlier.
Reactive Hypoglycemia in People Without Diabetes
If you don’t have diabetes and your blood sugar still drops, the most likely explanation is reactive hypoglycemia. This happens one to four hours after eating, typically after a meal heavy in refined carbohydrates. Your body overreacts to the rapid spike in blood sugar by releasing too much insulin, which then drives glucose levels below normal.
The exact mechanism isn’t always clear. The Mayo Clinic notes that symptoms are often connected to what and when a person eats, but doctors can’t always pinpoint a specific cause. Eating smaller, more frequent meals that combine protein, fat, and complex carbohydrates instead of simple sugars tends to prevent these post-meal crashes.
Bariatric Surgery Changes How You Process Food
Gastric bypass and similar weight-loss surgeries can cause blood sugar drops that start months or even years after the procedure. The reason is mechanical: after surgery, food bypasses the stomach and empties rapidly into the small intestine. This causes glucose to be absorbed much faster than normal, creating a sharp spike. Your intestine responds by releasing large amounts of a signaling hormone that tells your pancreas to produce insulin. The result is an insulin surge that overshoots, pulling blood sugar down well below where it started.
Several other factors compound this effect. People who’ve had bariatric surgery often have increased insulin sensitivity, meaning each unit of insulin is more powerful. They may also have a weaker glucagon response, the hormone that normally signals your liver to release stored sugar when levels drop. Together, these changes make post-meal lows a recurring problem for some surgical patients.
Hormonal Conditions That Lower Blood Sugar
Cortisol, the hormone your adrenal glands produce, plays a direct role in keeping blood sugar stable. It signals the liver to release glucose and helps counteract insulin’s effects. When your adrenal glands don’t produce enough cortisol (a condition called adrenal insufficiency), your body loses a key safety net against falling blood sugar. People with this condition also produce less adrenaline, which further weakens the body’s ability to bounce back from a low.
Growth hormone deficiency can cause similar problems. Growth hormone is one of several “counter-regulatory” hormones that work together to raise blood sugar when it drops. Tumors affecting the pancreas, though rare, can also cause persistent low blood sugar by producing excess insulin. Any unexplained, recurring episodes of low blood sugar warrant investigation into these hormonal causes.
Skipping Meals and Prolonged Fasting
Your body stores glucose in the liver as glycogen, and those reserves are limited. They can sustain you for roughly 12 to 24 hours depending on your activity level. If you skip meals, fast for extended periods, or exercise intensely without eating, those stores run out. Once glycogen is depleted, your blood sugar depends entirely on your liver’s ability to manufacture new glucose from protein and fat, a slower process that can’t always keep pace with demand.
This is why people who crash diet, do prolonged intermittent fasting without building up gradually, or combine heavy exercise with calorie restriction sometimes experience shakiness, lightheadedness, and difficulty concentrating. These are classic symptoms of blood sugar dropping below the level your brain needs to function normally.
How Low Blood Sugar Is Classified
The American Diabetes Association uses three levels. Level 1 is a reading between 54 and 69 mg/dL, which is below normal but usually manageable on your own. Level 2 is below 54 mg/dL, the point where your brain starts running short on fuel and you may experience confusion, blurred vision, or difficulty speaking. Level 3 is any episode severe enough that you need someone else to help you recover, regardless of the specific number on the meter.
The standard approach to treating a mild or moderate low is the 15-15 rule recommended by the CDC: eat or drink 15 grams of fast-acting carbohydrates (about four glucose tablets, half a cup of juice, or a tablespoon of honey), wait 15 minutes, and check your blood sugar again. If it’s still under 70 mg/dL, repeat the process.
When Your Body Stops Warning You
One of the more dangerous complications of repeated low blood sugar is that your body eventually stops sounding the alarm. Normally, when glucose drops, your brain triggers adrenaline release, which produces the shaking, sweating, and rapid heartbeat that tell you something is wrong. But when lows happen frequently, the brain adapts. It resets its threshold for what counts as “low,” essentially learning to tolerate glucose levels that should trigger a warning.
At the same time, the hormonal response weakens. Adrenaline, glucagon, and growth hormone all respond less aggressively after repeated episodes. The brain increases levels of an inhibitory chemical that dampens the signals from the region responsible for detecting low glucose. The result is that blood sugar can plummet to dangerous levels without any noticeable symptoms, a condition called hypoglycemia unawareness.
Interestingly, this adaptation behaves like a learned response, and it can be partially reversed. Strictly avoiding low blood sugar episodes for several weeks can help restore the body’s ability to detect and respond to drops. Sleep also plays a role: research has shown that sleep deprivation actually slowed the brain’s adaptation to repeated lows, suggesting that the process of “getting used to” hypoglycemia depends on normal sleep-based memory consolidation.