Hypoglycemia – blood glucose below 70 mg/dL – is most commonly caused by diabetes medications (especially insulin and sulfonylureas) combined with skipped meals, unexpected exercise, or alcohol. Other triggers include certain illnesses, hormonal problems, and, in people without diabetes, reactive drops after high-carb meals or after bariatric surgery.
How Hypoglycemia Happens
Your brain runs almost entirely on glucose and consumes about 120 grams a day. To keep levels steady, insulin lowers blood sugar after meals while glucagon, cortisol, and epinephrine raise it during fasting. Hypoglycemia occurs whenever this balance tips too far toward lowering – either because glucose-lowering drugs push too hard, the counter-regulatory hormones fail, or glucose supply runs out.
Symptoms begin around 70 mg/dL and worsen as glucose falls: shakiness, sweating, hunger, and irritability first; then confusion, slurred speech, and eventually seizures or loss of consciousness below about 40 mg/dL.
The Top Cause: Diabetes Medications
Insulin
Insulin is the most common cause of severe hypoglycemia. Risks rise with:
- Taking too large a mealtime dose for the carbs eaten
- Miscounting carbs and over-covering the meal
- Injecting rapid-acting insulin then delaying the meal
- Stacking doses (another correction on top of one that has not finished working)
- Injecting into muscle rather than fat, which speeds absorption
- Exercising within a few hours of a dose
Sulfonylureas
Sulfonylureas like glipizide, glyburide, and glimepiride force the pancreas to release insulin regardless of blood sugar. They are the oral drug class most likely to cause lows, especially in older adults and people with kidney disease. Glyburide has the highest risk.
Meglitinides and Others
Meglitinides (repaglinide, nateglinide) can trigger hypoglycemia if taken without a meal. GLP-1 agonists and SGLT2 inhibitors rarely cause lows alone but increase risk when combined with insulin or sulfonylureas. Metformin, DPP-4 inhibitors, and thiazolidinediones essentially do not cause hypoglycemia on their own.
Skipping or Delaying Meals
If you take medications that match insulin to an expected meal and the meal never arrives – or arrives hours late – insulin keeps working while glucose is not being absorbed. The result is a predictable low. This is one of the most common scenarios in hospital and outpatient settings.
Traveling, long meetings, religious fasting, and intermittent fasting all require medication adjustment for people on insulin or sulfonylureas. Never skip meals while on these drugs without first talking to your clinician.
Exercise and Physical Activity
Exercise increases insulin sensitivity and muscle glucose uptake. Hypoglycemia during or after activity is common because:
- Working muscles pull glucose out of the blood
- Insulin absorption from a recent injection accelerates
- Glycogen stores get depleted, and replenishment continues for up to 24 hours afterward (the “lag effect”)
Late-night lows are especially common after afternoon or evening exercise. See our exercise guide for specifics on timing and snack strategies.
Alcohol
Alcohol is a hidden major cause of hypoglycemia. When you drink, your liver prioritizes clearing alcohol over producing glucose. The glycogen-to-glucose pathway is suppressed for hours. On an empty stomach or with insulin or sulfonylureas, blood sugar can crash – often 6 to 24 hours after the drink, including during sleep.
Glucagon also works poorly when the liver is still processing alcohol, making alcohol-related lows harder to reverse. Guidelines suggest eating carbs with any alcoholic drink and checking glucose before bed.
Reactive (Post-Meal) Hypoglycemia
Reactive hypoglycemia happens 2 to 4 hours after eating, usually after a high-carb meal. The pancreas releases too much insulin for the glucose load, and sugar drops below normal once food absorption ends. It is most common in:
- People after gastric bypass or sleeve surgery (dumping syndrome)
- Early-stage insulin resistance and prediabetes
- Some people with no known underlying condition
Managing it centers on smaller, balanced meals with protein, fat, and fiber – exactly the pattern in our prediabetes diet guide.
Medical Conditions That Cause Hypoglycemia
| Condition | Mechanism |
|---|---|
| Adrenal insufficiency (Addison’s) | Low cortisol impairs glucose production |
| Pituitary failure | Low ACTH and growth hormone |
| Severe liver disease | Loss of glycogen storage and gluconeogenesis |
| Advanced kidney disease | Impaired drug clearance, reduced renal gluconeogenesis |
| Insulinoma (rare tumor) | Autonomous insulin secretion |
| Sepsis | Increased glucose consumption, liver dysfunction |
| Anorexia or prolonged fasting | Depleted glycogen stores |
| Autoimmune hypoglycemia | Antibodies against insulin or its receptor |
Drugs Besides Diabetes Medications
Several non-diabetes medications can trigger lows, particularly in vulnerable patients:
- Quinine (for malaria or leg cramps)
- Pentamidine
- Fluoroquinolone antibiotics (gatifloxacin, levofloxacin)
- Beta-blockers (blunt symptoms rather than cause lows)
- High-dose salicylates
- Indomethacin
Hypoglycemia Unawareness
People who have frequent lows can lose the usual warning symptoms, a condition called hypoglycemia unawareness. The adrenaline response that causes shakiness and sweating weakens with repeated exposure, so glucose can drop dangerously low before any symptom appears. The treatment is strict avoidance of lows for two to three weeks, which typically restores symptom awareness.
How to Treat a Low
The standard response is the rule of 15:
- Check your glucose. If below 70 mg/dL (or you have symptoms), eat 15 grams of fast-acting carbs – 4 glucose tablets, 4 oz juice, or 1 tablespoon of honey.
- Wait 15 minutes.
- Recheck. If still below 70, repeat another 15 grams.
- Once you are back above 70, eat a small snack with protein if your next meal is more than an hour away.
For severe lows where someone cannot swallow safely, use glucagon (injection or nasal spray) and call 911. Our hypoglycemia treatment guide covers emergency steps in full.
Preventing Future Lows
- Eat at consistent times if you take insulin or sulfonylureas
- Check glucose before driving and before bed
- Adjust insulin for exercise or discuss adjustment rules with your clinician
- Never drink alcohol on an empty stomach
- Always carry fast-acting carbs (glucose tabs are most reliable)
- Wear medical ID and teach family to use glucagon
- If lows are frequent, ask about a CGM with low-glucose alerts
The Bottom Line
Most hypoglycemia comes from a mismatch between glucose-lowering medication and food, activity, or alcohol. Insulin and sulfonylureas are the main culprits; metformin, DPP-4 inhibitors, and SGLT2 drugs rarely cause lows on their own. Learn your personal triggers, treat with 15 grams of fast carbs, and get any unexplained or severe low evaluated by your clinician – repeated lows are a signal that your regimen needs adjusting.
