Low blood sugar is medically called hypoglycemia. The term comes from Greek roots meaning “under” (hypo), “sweet” (glyc), and “blood” (emia). It’s generally defined as a blood sugar level below 70 mg/dL (3.9 mmol/L), and a reading below 54 mg/dL (3.0 mmol/L) is considered serious enough to require immediate action.
What Hypoglycemia Feels Like
Symptoms of hypoglycemia fall into two categories based on what’s happening inside your body. The first set comes from your nervous system kicking into a stress response: sweating, shakiness, a racing heartbeat, anxiety, and sudden intense hunger. These are your body’s early warning signals, and most people notice them first.
The second set of symptoms happens when your brain itself isn’t getting enough fuel. These include weakness, dizziness, difficulty concentrating, confusion, blurred vision, and behavior that can look like intoxication to other people. In extreme cases, blood sugar that stays very low can lead to seizures, loss of consciousness, or coma. The brain relies almost entirely on glucose for energy, so it’s uniquely vulnerable when levels drop.
How Your Body Fights Back
Your body has a layered defense system designed to keep blood sugar from falling too far. The first responder is glucagon, a hormone released by the pancreas. Glucagon signals the liver to break down stored sugar and release it into the bloodstream, producing a rapid bump in blood glucose. At the same time, your sympathetic nervous system releases adrenaline and noradrenaline, which boost liver glucose output while also dialing back insulin production to stop blood sugar from dropping further.
If the drop is more severe, cortisol and growth hormone join in. These provide a slower, more sustained effect: cortisol triggers the liver to produce new glucose from non-sugar sources and shifts other tissues toward burning fat instead of sugar. Growth hormone does something similar by making cells less responsive to insulin, effectively keeping more glucose available in the blood. This whole cascade is why mild episodes often resolve on their own in healthy people, but it can malfunction in certain medical conditions.
Reactive vs. Fasting Hypoglycemia
In people without diabetes, hypoglycemia typically takes one of two forms. Reactive hypoglycemia (also called postprandial hypoglycemia) happens within a few hours after eating, especially after meals heavy in carbohydrates. It’s more common in people who are prediabetic and struggling to produce the right amount of insulin at the right time, and in people who’ve had stomach surgery that causes food to move into the small intestine too quickly.
Fasting hypoglycemia occurs after long stretches without food, often overnight. It has a wider range of causes: alcohol (especially binge drinking), serious illnesses affecting the liver, heart, or kidneys, certain medications, hormone deficiencies, and rarely, insulin-producing tumors of the pancreas. The distinction matters because the timing of your symptoms relative to meals helps narrow down the underlying cause.
Who Gets It Most Often
Hypoglycemia is most common in people with diabetes, particularly those taking insulin or medications that stimulate insulin production. Too high a dose, a skipped meal, or unexpected physical activity can all tip the balance. But it also occurs in people without diabetes for the reasons listed above.
Severe hypoglycemia, classified as a level 3 episode, is defined by an altered mental or physical state so significant that you need another person’s help to recover. This is the level at which emergency glucagon is used. The American Diabetes Association recommends that anyone at increased risk of episodes below 54 mg/dL have a glucagon rescue product available.
How Hypoglycemia Is Diagnosed
Doctors use a set of three criteria known as Whipple’s triad to confirm a true hypoglycemic disorder. All three must be present: symptoms consistent with hypoglycemia, a low blood sugar reading measured by a laboratory test (not a home glucose meter) while those symptoms are occurring, and resolution of symptoms once blood sugar is brought back up. Meeting all three rules out other conditions that can mimic the same symptoms.
The 15-15 Rule for Treatment
The standard approach for a mild to moderate episode is called the 15-15 rule. You eat or drink 15 grams of fast-acting carbohydrates, then wait 15 minutes and recheck your blood sugar. Examples of 15 grams include about four glucose tablets, half a cup of juice, or a tablespoon of honey. If your level is still below 70 mg/dL, you repeat the process.
This method works because simple carbohydrates enter the bloodstream quickly without requiring much digestion. Reaching for something with fat or protein (like a candy bar or peanut butter) slows absorption and delays recovery. Once your blood sugar stabilizes, following up with a small balanced meal or snack helps prevent another dip.
For severe episodes where someone is unconscious or unable to swallow safely, glucagon is the treatment of choice. It’s available as an injection or a nasal spray, and a bystander can administer it without medical training. This is why people at risk are encouraged to keep glucagon accessible and make sure the people around them know where it is and how to use it.