When glucose levels drop below 70 mg/dL, your body launches an escalating series of stress responses to push blood sugar back up. You’ll typically feel shaky, sweaty, and hungry first. If levels keep falling below 54 mg/dL, the brain starts losing its primary fuel source, and symptoms shift from uncomfortable to dangerous, including confusion, seizures, and loss of consciousness.
Why Your Brain Is the First to Suffer
The brain is the body’s biggest glucose consumer. It accounts for only about 2% of your body weight but burns roughly 20% of all glucose-derived energy. That works out to about 5.6 mg of glucose per 100 grams of brain tissue every minute, a constant and enormous demand. Unlike muscles, which can switch to burning fat when glucose runs low, the brain depends almost entirely on a steady glucose supply. When that supply drops, cognitive function deteriorates within minutes.
Your Body’s Built-In Recovery System
A healthy body doesn’t wait passively for you to eat something. As blood sugar dips, it triggers a cascade of hormones designed to release stored energy and manufacture new glucose.
The first responders are glucagon and adrenaline. Glucagon, released from the pancreas, signals the liver to break down its stored glycogen into glucose and dump it into the bloodstream. This happens fast. Adrenaline kicks in almost simultaneously: it triggers the liver to produce even more glucose, pulls building blocks from muscle and fat to create new glucose, and also slows down how quickly other tissues absorb glucose so more stays available for the brain.
If the drop persists, cortisol and growth hormone join in. These work more slowly, promoting the creation of new glucose from non-sugar sources like amino acids and glycerol. This process, called gluconeogenesis, becomes increasingly important the longer blood sugar stays low. After an overnight fast, for instance, the liver’s glycogen stores are largely depleted, and gluconeogenesis becomes the dominant way your body keeps glucose flowing.
What Low Blood Sugar Feels Like
Symptoms come in two distinct waves tied to two different mechanisms.
The first wave is your nervous system reacting to the hormonal alarm response. These symptoms include trembling, a pounding heart, anxiety, sweating, intense hunger, and tingling sensations. They feel unpleasant, but they serve a purpose: they’re your body’s warning signal to eat something immediately.
The second wave comes from the brain itself running short on fuel. These symptoms are more serious and include confusion, difficulty thinking clearly, weakness, fatigue, a strange sensation of warmth, and in severe cases, seizures or loss of consciousness. By this point, the body’s recovery system has either failed or been overwhelmed.
The Three Levels of Severity
The American Diabetes Association defines three clinical levels. Level 1 is a blood sugar between 54 and 69 mg/dL. This is where most people notice the early warning symptoms and can treat themselves by eating. Level 2 is anything below 54 mg/dL, which is considered clinically significant and marks the threshold where brain function starts to degrade. Level 3 is defined not by a specific number but by the situation: you’re so impaired mentally or physically that you need someone else to help you recover. This can mean you’re unable to swallow food safely, are having a seizure, or have lost consciousness.
What Happens During a Severe Episode
When blood sugar drops low enough and stays there, the consequences go beyond feeling bad. Research shows that seizures in early hypoglycemia are linked to shifts in brain chemistry: sodium and potassium flood into brain cells, increasing pressure inside the skull. As the episode continues, the brain’s energy-supplying molecules (glucose, lactate, and certain amino acids) become depleted. This depletion coincides with the onset of coma.
Prolonged, untreated severe hypoglycemia can cause permanent brain damage. Within minutes of complete glucose depletion, neurons begin dying. The longer the brain goes without fuel, the wider the area of damage spreads. This is why severe episodes are medical emergencies.
Why Some People Stop Feeling the Warnings
One of the most dangerous complications of repeated low blood sugar is losing the ability to sense it happening. This condition, called hypoglycemia unawareness, affects roughly 40% of people with type 1 diabetes. The more often blood sugar drops, the more the body recalibrates its alarm system downward, so the warning symptoms that used to kick in at 70 mg/dL may not appear until levels hit 50 or lower.
Several mechanisms drive this. Repeated lows increase levels of an inhibitory brain chemical in the hypothalamus, the brain region that normally sounds the alarm. This essentially mutes the signal. The brain also appears to adapt by finding alternative fuel sources during low glucose periods, which reduces the perceived urgency of each episode. The result is a vicious cycle: each unrecognized low makes the next one harder to detect, and the risk of a severe episode climbs.
The good news is that this process is partially reversible. Carefully avoiding hypoglycemia for several weeks can restore the body’s ability to detect and respond to low blood sugar.
Causes Beyond Diabetes
Diabetes medications are the most common cause of low blood sugar, but they’re far from the only one. In people without diabetes, low glucose can result from a wide range of conditions.
- Liver or kidney failure: The liver stores glycogen and manufactures new glucose. When it fails, both capacities drop. The kidneys also contribute to glucose production, and kidney failure impairs insulin clearance, letting insulin linger longer than it should.
- Adrenal or pituitary problems: The adrenal glands produce cortisol, which helps maintain blood sugar. When they fail, glucose production decreases and insulin sensitivity increases, a combination that pulls blood sugar down.
- Gastric bypass surgery: Some people develop low blood sugar after eating, likely because food moves through the digestive system faster than normal, triggering an outsized insulin release.
- Alcohol: Heavy drinking can suppress the liver’s ability to produce glucose, especially on an empty stomach.
- Insulinomas: These rare pancreatic tumors secrete insulin continuously, regardless of blood sugar levels.
- Certain medications: Some antibiotics, blood pressure drugs, and other common medications have been linked to hypoglycemia as a side effect.
How to Treat a Low in the Moment
The standard approach is the 15-15 rule recommended by the CDC: eat 15 grams of fast-acting carbohydrates, wait 15 minutes, then check your blood sugar again. If it’s still below 70 mg/dL, repeat the cycle. Keep going until levels return to your target range. Once they do, follow up with a balanced snack or small meal containing protein and carbohydrates to prevent another drop.
Fifteen grams of carbohydrates looks like about four glucose tablets, half a cup of juice, or a tablespoon of honey. The key is choosing something that absorbs quickly. A candy bar with a lot of fat, for example, will take much longer to raise blood sugar than pure glucose.
For severe episodes where someone is confused, seizing, or unconscious, they cannot safely eat or drink. This is when emergency glucagon, available as an injection or nasal spray, becomes necessary. Someone nearby needs to administer it and call for emergency help.