Low blood sugar, known medically as hypoglycemia, happens when your blood glucose drops below 70 mg/dL. The most common cause is diabetes medication, but dozens of other factors can trigger it in people with or without diabetes, from skipping meals to drinking alcohol to underlying organ disease. Understanding what’s behind a drop helps you prevent it or recognize when something deeper needs attention.
How Low Blood Sugar Is Classified
Not all episodes are equally serious. The American Diabetes Association breaks hypoglycemia into three levels. Level 1 is a reading between 54 and 69 mg/dL, which is low enough to notice but usually easy to correct on your own. Level 2 is anything below 54 mg/dL, the point where your brain starts running short on fuel and you may feel confused, clumsy, or unable to think clearly. Level 3 is any episode severe enough that you need someone else’s help to recover, regardless of the exact number on a meter.
For people without diabetes, the threshold that raises clinical suspicion is slightly lower. Doctors often look for blood sugar below 55 mg/dL combined with symptoms that improve after eating, a pattern sometimes called Whipple’s triad.
Diabetes Medications
Insulin is the single most common trigger. Taking more than your body needs at that moment, whether because of a dosing mistake or a change in routine, pushes blood sugar down fast. This applies to people with type 1 or type 2 diabetes who inject insulin.
A class of oral medications called sulfonylureas (brand names include Glucotrol, Amaryl, and Diabeta) also carries significant risk because these drugs stimulate your pancreas to release insulin around the clock, not just when blood sugar is high. Other oral combinations can compound the problem. Metformin paired with a sulfonylurea, or certain newer glucose-lowering drugs taken alongside a sulfonylurea, increases the chance of a low.
The most straightforward trigger in this category is simply eating less than expected or eating later than usual after taking your medication. If you’ve already dosed insulin or taken a sulfonylurea, skipping a meal or delaying it by even an hour or two can leave too much medication working against too little food.
Alcohol and the Liver
Your liver is your body’s glucose factory. Between meals and overnight, it produces new glucose through a process called gluconeogenesis to keep your blood sugar stable. Alcohol directly interferes with that process. In one study of healthy, fasting men, a single dose of alcohol cut the liver’s glucose production by 45% over five hours and reduced the raw materials the liver needs for glucose production by 61%.
This is why drinking on an empty stomach, or drinking heavily without eating, is a reliable recipe for low blood sugar. The effect can hit hours after your last drink, catching people off guard the next morning. If you take insulin or sulfonylureas, alcohol multiplies the risk because the medication is still pulling blood sugar down while the liver’s ability to compensate is impaired.
Exercise and Physical Activity
Working muscles pull glucose out of your bloodstream for energy, which is one reason exercise is so effective for blood sugar management. But that same mechanism can overshoot. Physical activity makes your cells more responsive to insulin for up to 24 hours after your workout, meaning blood sugar can drop not only during exercise but well into the next day.
This delayed effect is called late-onset post-exercise hypoglycemia, and it’s particularly common after unusually long or intense activity. People with diabetes sometimes experience overnight lows after an active afternoon. Even people without diabetes can notice symptoms after prolonged endurance exercise if they haven’t fueled adequately.
Skipping Meals and Undereating
Your body stores a backup supply of glucose in the liver and muscles as glycogen. When you skip meals or go long stretches without eating, your body draws down those stores. Once they’re depleted, blood sugar can fall. Short-term fasting rarely causes problems in healthy people because the liver compensates, but prolonged starvation or severe calorie restriction can overwhelm that backup system.
Eating disorders present a serious risk. Anorexia nervosa, for example, can deplete glycogen stores entirely, leaving the body unable to maintain normal blood sugar. Malnutrition from any cause, whether an eating disorder, chronic illness, or food insecurity, can do the same.
Reactive Hypoglycemia After Meals
Some people experience blood sugar drops within four hours of eating, a pattern called reactive or postprandial hypoglycemia. This happens when the body releases too much insulin in response to a meal, driving blood sugar below normal after the food has been absorbed.
Reactive hypoglycemia is especially common after gastric bypass and other bariatric surgeries. When the stomach is smaller or rerouted, food reaches the intestine faster, triggering a larger and quicker insulin surge. Certain inherited metabolic conditions can also cause this pattern. For many people, though, the exact mechanism is never pinpointed. Eating smaller, more frequent meals with a mix of protein, fat, and complex carbohydrates typically reduces the severity.
Organ Disease and Critical Illness
Several serious medical conditions can cause low blood sugar independent of diabetes or medication.
- Liver disease: Because the liver is responsible for producing and storing glucose, severe hepatitis or cirrhosis can cripple its ability to keep blood sugar stable, especially during fasting or overnight.
- Kidney disease: The kidneys help clear medications from your body. When they aren’t working well, drugs that lower blood sugar can accumulate and have a stronger, longer-lasting effect. Kidney disease also directly impairs glucose regulation.
- Advanced heart disease and severe infections: Critical illness of any kind raises the body’s demand for glucose while sometimes reducing the ability to produce it. Sepsis and other overwhelming infections are particularly notorious for causing dangerous lows.
These causes typically show up alongside other obvious symptoms of the underlying disease. Hypoglycemia that appears in someone who is already hospitalized or visibly unwell points to one of these conditions rather than a dietary or medication issue.
Hormone Deficiencies
Cortisol, produced by the adrenal glands, plays a key role in keeping blood sugar from dropping too low. It signals the liver to release stored glucose and helps counterbalance insulin. In Addison’s disease, the adrenal glands don’t produce enough cortisol, and low blood sugar is an early and sometimes dangerous symptom. During an adrenal crisis, blood sugar can plummet alongside blood pressure.
A related problem, secondary adrenal insufficiency, occurs when the pituitary gland in the brain fails to send the right signals to the adrenal glands. People with this condition are even more prone to low blood sugar because the hormone chain that protects against glucose drops is broken at a higher level. Growth hormone deficiency, which can also stem from pituitary problems, contributes in a similar way.
Insulin-Producing Tumors
An insulinoma is a rare, usually benign tumor in the pancreas that secretes insulin continuously, regardless of what your blood sugar is doing. The result is repeated episodes of hypoglycemia that seem to come out of nowhere, often during fasting or between meals. These tumors are uncommon, but they’re an important consideration when someone without diabetes has unexplained, recurring lows.
Diagnosis typically involves a supervised fast lasting up to 72 hours in a hospital setting. Doctors wait for blood sugar to drop below 55 mg/dL while measuring insulin and related markers to confirm that the pancreas is producing insulin inappropriately. Most insulinomas can be surgically removed.
Non-Diabetes Medications
A number of drugs prescribed for conditions unrelated to diabetes can lower blood sugar as a side effect. Some of the more common culprits include certain antibiotics (particularly fluoroquinolones like levofloxacin, and the combination antibiotic trimethoprim-sulfamethoxazole), the anti-malaria drug quinine, the pain reliever indomethacin, and heart rhythm medications like quinidine. Beta-blockers, used for high blood pressure and heart conditions, can mask the typical warning signs of a low (like a racing heart) while also contributing to the drop itself.
If you notice symptoms of low blood sugar after starting a new medication, that connection is worth investigating. The risk increases when these drugs are combined with each other or taken by someone with kidney impairment, which slows their clearance from the body.