Blood glucose drops below normal when your body either uses too much glucose, produces too little, or releases too much insulin. A reading below 70 mg/dL is considered low, and anything below 54 mg/dL is classified as severely low. The causes range from something as simple as a skipped meal to underlying hormonal or organ problems that need medical attention.
How Your Body Normally Keeps Glucose Stable
Your blood sugar is managed by a balancing act between hormones. When you eat, your pancreas releases insulin to move glucose from your blood into your cells. Between meals and overnight, a different hormone called glucagon signals your liver to release stored glucose so your levels don’t crash. Several gut hormones fine-tune this system further by adjusting how much insulin and glucagon get released after a meal.
If glucose keeps falling despite glucagon, your body has backup systems. The adrenal glands release cortisol and adrenaline, both of which push the liver to produce more glucose. Growth hormone from the brain does something similar. These backup hormones typically kick in when glucose drops to around 58 to 66 mg/dL. When any part of this system fails or gets overwhelmed, glucose can fall too low.
Diabetes Medication Is the Most Common Cause
If you take insulin or certain oral diabetes medications, you’re at the highest risk for low blood sugar. Taking more insulin than your body needs at that moment is the single most frequent trigger. This can happen when you miscalculate a dose, take your medication at the wrong time, or take it without eating enough food afterward.
Other common triggers in people with diabetes include:
- Skipping or delaying meals. Your medication keeps working whether you eat or not.
- Exercising more than usual. Physical activity burns through glucose faster, and if your medication dose stays the same, your blood sugar can drop during or after a workout.
- Drinking alcohol. Alcohol directly interferes with your liver’s ability to produce glucose (more on this below).
People with type 1 diabetes face an additional challenge. Their pancreatic beta cells are destroyed, which means they don’t produce insulin or a companion hormone called amylin. Without amylin, glucagon isn’t properly suppressed after meals and isn’t properly activated when blood sugar falls. This makes glucose levels harder to predict and control in both directions.
Low Glucose After Eating (Reactive Hypoglycemia)
Some people experience a glucose drop within four hours of eating a meal, even without diabetes. This is called reactive hypoglycemia, and it typically happens after meals high in refined carbohydrates. The body overreacts to the sugar rush by releasing too much insulin, which then drives blood sugar below normal.
In many cases, no clear underlying cause is found. But certain situations make reactive drops more likely. People who have had gastric bypass or other bariatric surgery are especially prone to it because food moves into the small intestine faster, triggering a larger insulin response. Rare inherited metabolic conditions and certain pancreatic tumors can also cause it.
Why Alcohol Causes Glucose to Drop
Alcohol has a surprisingly powerful effect on blood sugar because it disrupts your liver’s ability to make new glucose from scratch, a process called gluconeogenesis. In one study of overnight-fasted men, alcohol consumption reduced the liver’s access to glucose-building materials by about 61%. Individual steps in the glucose-production pathway were inhibited by 60 to 75%.
Your liver partially compensates by tapping into its stored glucose reserves, but those reserves are limited, especially if you haven’t eaten recently. This is why drinking on an empty stomach or after prolonged fasting is particularly risky. For people taking insulin or diabetes medications, alcohol amplifies the glucose-lowering effect and can cause dangerous drops hours after drinking, sometimes even during sleep.
Causes in People Without Diabetes
Low blood sugar in someone who doesn’t have diabetes is less common, but it does happen. Doctors look for a combination of three things to confirm it: a low glucose reading, symptoms consistent with low glucose, and improvement in those symptoms once glucose rises. If all three are present, further testing is usually warranted.
Hormonal Deficiencies
Your body’s glucose backup system depends on cortisol and growth hormone. If your adrenal glands don’t produce enough cortisol (a condition called adrenal insufficiency) or your pituitary gland doesn’t release enough growth hormone, your body can’t mount a proper defense against falling blood sugar. Research on people with pituitary disorders has shown they reach significantly lower glucose levels than healthy individuals during fasting, precisely because these counterregulatory hormones are missing.
Insulinomas
An insulinoma is a tumor in the pancreas that produces insulin continuously, regardless of whether your blood sugar needs lowering. Normally, the pancreas stops making insulin when glucose falls. These tumors override that safety mechanism. Symptoms tend to be worst when fasting, skipping meals, or exercising, since those are the times your body most needs glucose production to go uninterrupted.
Medications That Aren’t for Diabetes
Several non-diabetes drugs can lower blood sugar as a side effect. These include certain beta-blockers used for blood pressure and heart conditions, some heart rhythm medications, a common anti-inflammatory pain reliever (indomethacin), and several antibiotics including fluoroquinolones and trimethoprim-sulfamethoxazole. Beta-blockers can also mask the early warning symptoms of low blood sugar, making it harder to recognize a drop before it becomes severe.
Organ Dysfunction
Severe liver disease impairs glucose production because the liver is the primary organ responsible for releasing stored glucose and manufacturing new glucose between meals. Kidney disease can also contribute because the kidneys play a supporting role in glucose production and help clear insulin from the bloodstream. When the kidneys aren’t functioning well, insulin (whether natural or injected) lingers longer than it should.
How Low Glucose Feels at Different Stages
The symptoms of falling blood sugar follow a predictable sequence. Early on, your nervous system fires off warning signals: sweating, a racing heart, shaking hands, anxiety, and sudden hunger. These symptoms act as an alarm system, and most people learn to recognize them.
If glucose continues to drop, the brain itself starts running short on fuel. At this stage, symptoms shift from physical to neurological: confusion, difficulty concentrating, irritability, slurred speech, and impaired coordination. In severe cases, this can progress to hallucinations, seizures, loss of consciousness, and coma. The physical warning symptoms almost always come first, giving you time to eat or drink something sugary. But in people who experience frequent low blood sugar episodes, particularly those with long-standing diabetes, the early warning system can become dulled over time, which means the first noticeable symptoms may already be the more dangerous neurological ones.
Patterns That Point to the Cause
The timing of your low glucose episodes tells a lot about what’s driving them. Drops that happen two to four hours after meals suggest reactive hypoglycemia or medication timing issues. Drops that happen after fasting, overnight, or when you skip meals point toward problems with glucose production, such as an insulinoma, liver disease, or hormonal deficiency. Drops tied to exercise suggest either medication-related causes or inadequate fuel intake before activity. Tracking when your episodes happen, what you ate beforehand, and what medications you took gives your doctor the clearest picture of what’s going wrong.