Blood sugar can drop for many reasons, from skipping a meal to drinking alcohol to taking certain medications. A reading below 70 mg/dL is generally considered low, and at that level most people start feeling shaky, lightheaded, or irritable. Some causes are everyday and easy to fix, while others point to underlying health issues worth investigating.
Skipping Meals or Not Eating Enough
The most common and straightforward cause is simply not eating for too long. Your body maintains blood sugar between meals by tapping into glucose stored in the liver, but those reserves are limited. If you skip breakfast, fast for an extended stretch, or eat a meal that’s too small, your liver’s supply can run low before your next meal arrives. This is especially true if you’re also physically active during that window.
Eating Patterns That Trigger a Sugar Crash
Ironically, eating the wrong things can cause your blood sugar to drop sharply. This is called reactive hypoglycemia, and it typically hits within four hours after a meal. Here’s what happens: you eat something high in sugar or refined carbohydrates (white bread, candy, soda), your blood sugar spikes quickly, and your pancreas releases a large burst of insulin to bring it down. Sometimes that insulin response overshoots, pulling your blood sugar lower than where it started.
Sugary foods and processed simple carbohydrates eaten on an empty stomach are the most common triggers. The fix is pairing carbohydrates with protein, fat, or fiber to slow digestion and prevent that rapid spike-and-crash cycle.
Alcohol’s Effect on Blood Sugar
Alcohol is one of the more surprising causes. When you drink, your liver prioritizes breaking down the alcohol over its other jobs, including releasing stored glucose into your bloodstream. Research published in the American Journal of Physiology found that alcohol reduced the liver’s glucose production by 45% in the five hours after drinking. That’s a significant drop in one of your body’s main mechanisms for keeping blood sugar stable.
The risk is highest when you drink on an empty stomach or after exercise, since your liver’s glucose reserves are already low in those situations. The effect can be delayed too, meaning your blood sugar might not drop until hours after your last drink, sometimes even while you’re sleeping.
Exercise and Physical Activity
During exercise, your muscles burn through glucose as fuel, which naturally pulls sugar out of your bloodstream. Prolonged or intense exercise can also deplete the glucose stored in your liver, reducing its ability to top off your blood sugar levels.
What catches people off guard is delayed drops that happen hours after a workout. Exercise increases your body’s sensitivity to insulin, and that heightened sensitivity can persist well into the evening or even overnight. Your muscles continue pulling in glucose to replenish their stores long after you’ve stopped moving. This is why blood sugar can dip unexpectedly in the middle of the night following an intense afternoon workout.
Medications That Lower Blood Sugar
Diabetes medications are the most obvious culprit here, particularly insulin and drugs that stimulate insulin production. But several non-diabetes medications can also cause unexpected drops:
- Beta-blockers used for blood pressure and heart conditions (like atenolol and propranolol)
- Certain antibiotics, including some fluoroquinolones and trimethoprim-sulfamethoxazole
- Quinine, sometimes used for malaria or leg cramps
- Some anti-inflammatory pain relievers, such as indomethacin
- Heart rhythm medications like quinidine
Beta-blockers are particularly tricky because they can also mask the typical warning signs of low blood sugar, like a racing heart and shakiness. If you’re on any of these medications and experiencing symptoms of low blood sugar, it’s worth bringing up with your prescriber.
Hormonal Imbalances
Two hormones play critical roles in keeping blood sugar from falling too low: cortisol (produced by the adrenal glands) and growth hormone (produced by the pituitary gland). When your blood sugar starts to dip, your body releases these hormones to trigger the liver to produce more glucose and to reduce how quickly your cells absorb the glucose that’s already circulating.
People with adrenal insufficiency don’t produce enough cortisol, which removes a key safety net. Without adequate cortisol, the body becomes more sensitive to insulin, cells take up glucose faster, and the liver produces less new glucose. Growth hormone deficiency creates a similar problem, and people who lack both hormones face an even greater risk of low blood sugar episodes. These conditions are relatively uncommon but can cause persistent, hard-to-explain drops that don’t respond to simple dietary changes.
Kidney Disease
The kidneys play a larger role in blood sugar regulation than most people realize. They’re responsible for clearing and breaking down about 25% of the insulin your pancreas produces each day. When kidney function declines, insulin stays active in the bloodstream longer than it should, continuing to push blood sugar down after it would normally be cleared.
This effect is even more pronounced in people with diabetes who inject insulin. Normally, insulin produced by the pancreas passes through the liver first, where a significant portion is filtered out before it reaches the rest of the body. Injected insulin skips the liver entirely and enters the general circulation directly, making the kidneys an even more important part of the cleanup process. When those kidneys aren’t working well, the injected insulin lingers and its blood sugar-lowering effect is amplified.
Stomach and Bariatric Surgery
People who’ve had gastric bypass or other stomach surgeries can experience a condition called late dumping syndrome. Because the stomach has been restructured, food (especially sugary or starchy meals) passes into the small intestine much faster than normal. This triggers a rapid spike in blood sugar, followed by an oversized insulin response that drives blood sugar down sharply. The drop typically hits one to three hours after eating.
The pattern looks similar to reactive hypoglycemia, but the mechanism is more extreme because of the altered anatomy. Eating smaller, more frequent meals that are lower in simple sugars is the primary way people manage this.
Insulin-Producing Tumors
Rarely, a small tumor on the pancreas called an insulinoma can produce excess insulin regardless of what you eat or how active you are. The hallmark pattern is low blood sugar that occurs during fasting, most often in the morning after an overnight fast. Doctors look for three signs together: symptoms of low blood sugar, a confirmed reading below 55 mg/dL, and improvement of symptoms after eating carbohydrates. Most insulinomas are benign and can be surgically removed.
What To Do When Blood Sugar Drops
The standard approach recommended by the CDC is the 15-15 rule: consume 15 grams of fast-acting carbohydrates (four glucose tablets, half a cup of juice, or a tablespoon of honey), then wait 15 minutes and recheck. If your blood sugar is still below 70 mg/dL, repeat the process. Once it’s back to normal, eat a balanced snack or meal to keep it stable.
If blood sugar drops are happening regularly and you can’t trace them to an obvious cause like missed meals, alcohol, or exercise, that pattern is worth investigating. Recurring episodes can point to medication effects, hormonal deficiencies, or other conditions that are treatable once identified.