Low blood sugar, or hypoglycemia, is a blood glucose reading below 70 mg/dL. It causes shakiness, sweating, confusion, and in severe cases unconsciousness. Most mild lows can be reversed in 15 minutes with a fast-acting carbohydrate, but severe hypoglycemia is a medical emergency.
What Counts as Low Blood Sugar
The American Diabetes Association classifies hypoglycemia by severity. The numbers matter because treatment changes as glucose drops.
| Level | Glucose Reading | What It Means |
|---|---|---|
| Level 1 (Mild) | 54-69 mg/dL | Alert; treat with 15 g fast carbs. |
| Level 2 (Moderate) | Less than 54 mg/dL | Clinically significant; act fast. |
| Level 3 (Severe) | Any reading with altered mental or physical status requiring help | Glucagon and 911. |
According to the CDC, hypoglycemia is most common in people taking insulin or sulfonylureas, but it can occur in anyone under the right conditions.
What Causes Low Blood Sugar
Diabetes Medications
Insulin and sulfonylureas (glipizide, glyburide, glimepiside) are the most common medication culprits. Taking too much, mistiming the dose, or pairing with a smaller-than-usual meal can drop glucose quickly. Metformin alone rarely causes lows.
Skipped or Delayed Meals
If you take medication that lowers blood sugar but skip the meal it was meant to cover, glucose will fall. Even without medication, going many hours without eating can cause reactive lows in people with insulin resistance.
Alcohol
Alcohol blocks the liver’s ability to release stored glucose. Drinking on an empty stomach or after exercise can trigger hypoglycemia hours later, often overnight.
Exercise
Muscles pull glucose from the bloodstream during and for hours after activity. Unplanned exercise without adjusting medication or food is a frequent cause of post-workout lows.
Other Causes
- Hormone disorders (adrenal insufficiency, hypopituitarism)
- Severe liver, kidney, or heart disease
- Bariatric (weight-loss) surgery — can cause reactive hypoglycemia
- Insulinoma — a rare insulin-producing tumor
- Drugs such as quinine, pentamidine, beta-blockers
Symptoms by Severity
| Severity | Glucose Range | Symptoms |
|---|---|---|
| Mild | 54-69 mg/dL | Shakiness, sweating, hunger, fast heartbeat, anxiety, tingling lips |
| Moderate | 40-54 mg/dL | Confusion, blurred vision, slurred speech, weakness, irritability, headache |
| Severe | Below 40 mg/dL | Inability to swallow, seizure, loss of consciousness, coma |
Some people develop hypoglycemia unawareness, where they no longer feel mild symptoms. This is dangerous because the first warning may be confusion or fainting. For more on warning signs see our companion guide on symptoms to watch.
How to Treat Low Blood Sugar: The 15-15 Rule
For mild to moderate lows in a person who is awake and able to swallow:
- Confirm with a meter if possible.
- Eat or drink 15 grams of fast-acting carbohydrate.
- Wait 15 minutes.
- Recheck. If still under 70 mg/dL, repeat with another 15 grams.
- Once above 70 mg/dL, eat a small balanced snack if your next meal is more than an hour away.
15-Gram Fast-Carb Examples
- 4 glucose tablets (preferred — measured dose)
- 1 tube of glucose gel
- 4 oz (½ cup) of juice or regular soda
- 1 tablespoon of honey, sugar, or syrup
- 5-6 hard candies (not chocolate — fat slows absorption)
- 1 cup of skim milk
Avoid chocolate, peanut butter, and other fatty foods first — fat delays sugar absorption when you need it fast.
Treating Severe Hypoglycemia
If a person is unconscious, seizing, or unable to swallow safely, do not give food or drink. Instead:
- Call 911.
- Administer glucagon if a kit is available — injectable, nasal spray (Baqsimi), or pre-filled pen (Gvoke).
- Turn the person on their side to protect the airway.
- Stay with them until help arrives or they are alert.
- Once awake, give 15 g of carbs followed by a snack with protein.
Anyone using insulin or a sulfonylurea should have a glucagon prescription on hand and family members trained to use it, per the American Diabetes Association.
How to Prevent Low Blood Sugar
- Eat consistently — don’t skip meals if you take glucose-lowering medication.
- Match insulin to carbs — count carbs and match the dose, especially with rapid-acting insulin.
- Test before driving — and before sports or strenuous activity.
- Carry fast carbs at all times — glucose tablets in a pocket, purse, or car.
- Avoid alcohol on an empty stomach — and check glucose before bed if you’ve had a drink.
- Wear medical ID — bracelet or app on your phone’s lock screen.
- Use a CGM — alarms catch falling glucose before symptoms appear.
Hypoglycemia Without Diabetes
Reactive hypoglycemia happens 2-4 hours after eating, often after a high-carb meal. The pancreas overshoots insulin release, and glucose dips below 70 mg/dL. Symptoms mirror diabetic hypoglycemia. Management focuses on smaller, balanced meals with fiber and protein.
Fasting hypoglycemia (low sugar after going hours without food) is rarer and more serious. It can suggest hormone deficiency, severe organ disease, or insulinoma, and warrants a workup with your doctor.
When to Call Your Doctor
- You have lows more than once or twice a week.
- You no longer feel early warning symptoms (hypoglycemia unawareness).
- You needed someone else’s help to recover from a low.
- You used glucagon.
- You are not sure what caused the low.
Your doctor may adjust your treatment plan, change meal timing, or recommend a continuous glucose monitor.
The Bottom Line
Low blood sugar is a glucose reading below 70 mg/dL and is most common in people on insulin or sulfonylureas. The 15-15 rule reverses most mild lows: 15 g of fast carbs, wait 15 minutes, recheck. Severe hypoglycemia with confusion or unconsciousness requires glucagon and 911 — never put food in the mouth of someone who cannot safely swallow. Carry fast-acting carbs, test before driving, and talk to your doctor if lows are happening more than occasionally.
