Blood sugar drops when your body uses or stores glucose faster than it can replenish it. In healthy people, this rarely happens because insulin and glucagon work together to keep blood sugar steady. But medications, alcohol, prolonged exercise, skipped meals, and certain medical conditions can all tip that balance. A reading below 70 mg/dL is generally considered low blood sugar, or hypoglycemia.
How Your Body Normally Controls Blood Sugar
After you eat, glucose enters your bloodstream and triggers your pancreas to release insulin. Insulin does two things: it helps cells absorb glucose for energy, and it tells your liver to stop releasing its stored glucose. As blood sugar falls back toward normal, insulin production tapers off and your pancreas starts releasing glucagon instead. Glucagon signals your liver to break down its glycogen stores and release glucose back into the blood. This back-and-forth between insulin and glucagon keeps your blood sugar in a narrow range throughout the day.
When something disrupts this cycle, whether too much insulin, not enough glucagon, or a liver that can’t release glucose fast enough, blood sugar drops.
Diabetes Medications Are the Most Common Cause
Low blood sugar is uncommon in people who don’t take glucose-lowering drugs. For people with diabetes, though, it’s one of the most frequent complications of treatment. Insulin injections and certain oral medications (particularly sulfonylureas) lower blood sugar by design, and they can overshoot the target if circumstances change.
The most common triggers in people on these medications are straightforward: skipping or delaying a meal, eating less than expected, exercising more than usual, or accidentally taking too much medication. Each of these creates a mismatch where the drug pushes blood sugar down but there isn’t enough incoming glucose to balance it out. Even with careful management, these mismatches are hard to avoid entirely.
How Exercise Pulls Sugar Out of Your Blood
When a muscle contracts, it opens glucose channels on its surface that pull sugar directly out of the bloodstream. These channels, normally tucked away inside the cell, move to the cell’s outer membrane during physical activity. This happens independently of insulin, which is why exercise can lower blood sugar even when insulin levels are low.
During short workouts, your liver compensates by releasing more glucose. But with prolonged exercise, the liver’s glycogen stores start running out. If the liver can’t produce new glucose fast enough to keep up with what muscles are burning, blood sugar drops. This is why endurance athletes sometimes “bonk” or “hit the wall” during long events, and why people with diabetes need to plan carefully around physical activity.
Regular exercise also has a longer-term effect: it increases the number of glucose channels your muscle cells produce, making them more sensitive to insulin for hours or even days after a workout. This improved sensitivity is beneficial for overall health, but it means your usual dose of diabetes medication might become too strong after a particularly active day.
Alcohol Suppresses Your Liver’s Backup System
Your liver has two ways to supply glucose between meals. It can break down stored glycogen, or it can build new glucose from scratch using smaller molecules like amino acids and lactate. Alcohol interferes with the second process. In one study of fasting men, alcohol reduced the liver’s ability to build new glucose by 45% and cut the availability of the raw materials needed for that process by 61%.
This matters most when you haven’t eaten in a while. If your glycogen stores are already low (say, after an evening of drinking without much food), your liver’s backup system is exactly what you’re relying on. Alcohol shuts it down right when you need it most. This is why drinking on an empty stomach, or drinking heavily without eating, can cause blood sugar to drop hours later, sometimes even the next morning.
Blood Sugar Crashes After Eating
Some people experience low blood sugar within four hours of a meal, a pattern called reactive hypoglycemia. What typically happens is that a meal high in sugar or refined carbohydrates causes a rapid spike in blood sugar, which triggers a large insulin release. The insulin then overshoots, driving blood sugar below where it started.
The exact cause isn’t always clear, but sugary foods, white bread, white pasta, and other processed carbohydrates eaten on an empty stomach are common triggers. People who have had gastric bypass or other weight-loss surgery are particularly prone to this because food moves through their digestive system faster, leading to quicker glucose absorption and a bigger insulin response.
Medical Conditions That Lower Blood Sugar
Outside of diabetes treatment, persistent low blood sugar usually points to an underlying medical issue. One of the more well-known causes is an insulinoma, a rare tumor in the pancreas that continuously pumps out insulin regardless of what your blood sugar is doing. Doctors suspect an insulinoma when someone has repeated low blood sugar episodes, documented readings below 55 mg/dL, and symptoms that improve after eating carbohydrates. About 5% to 10% of insulinoma cases are linked to an inherited condition called multiple endocrine neoplasia type 1.
Other conditions that can cause low blood sugar include adrenal insufficiency (where the adrenal glands don’t produce enough of the hormones that raise blood sugar), severe liver disease (which limits the liver’s ability to store and release glucose), and kidney failure (which affects how the body clears insulin). These are all relatively uncommon, but they’re worth investigating if low blood sugar keeps happening without an obvious explanation.
How Your Body Fights Back Against a Drop
When blood sugar starts falling, your body mounts a defense in a specific sequence. First, the pancreas dials back insulin production. Next, it releases glucagon to push stored glucose out of the liver. If blood sugar keeps falling, your adrenal glands release adrenaline, which is what causes the shaking, sweating, and rapid heartbeat that most people recognize as a “low.” Cortisol and growth hormone follow, both of which work to raise blood sugar over a longer timeframe. Finally, the brain generates hunger and anxiety to push you toward eating.
This layered response is remarkably effective in healthy people, which is why true hypoglycemia is rare without medication. But in people with diabetes who experience frequent lows, the system can break down.
Why Repeated Lows Dull the Warning Signs
When someone with diabetes has frequent episodes of low blood sugar, the body gradually recalibrates. The threshold at which adrenaline releases and warning symptoms appear shifts lower. Over time, the brain adapts to operating at lower glucose levels and stops sounding the alarm at 70 mg/dL. Blood sugar can then drop to dangerously low levels before the person feels anything wrong.
This creates a vicious cycle: each unrecognized low episode makes the next one harder to detect, and the lack of warning symptoms makes severe hypoglycemia more likely. The body’s reduced ability to mount a sympathetic nervous system response also impairs its ability to self-correct by releasing glucose from the liver. Avoiding lows for several weeks can partially reverse this pattern and restore the body’s normal warning signals.
Skipped Meals and Prolonged Fasting
Your liver stores enough glycogen to maintain blood sugar for roughly 12 to 24 hours without food, depending on your activity level and how full those stores were to begin with. After that, you depend entirely on gluconeogenesis, the process of building new glucose. In healthy people, this transition happens smoothly. But if something else is going on, like alcohol use, liver disease, or medications that suppress glucose production, skipping meals can tip you into hypoglycemia faster than you’d expect.
For people not on diabetes medication and without an underlying condition, simply skipping lunch won’t usually cause clinical hypoglycemia. You might feel lightheaded or irritable, but your blood sugar is unlikely to fall below 70 mg/dL. If it does, that’s a signal worth investigating.