A low glucose reading means your blood doesn’t have enough sugar to fuel your brain and body properly. For people with diabetes, low blood sugar (called hypoglycemia) is generally defined as a reading below 70 mg/dL. For people without diabetes, the threshold is lower, typically below 55 mg/dL. Whether this is a one-time event or a recurring pattern makes a big difference in what it means for your health and what to do next.
What Your Body Does When Glucose Drops
Your brain runs almost entirely on glucose, so when levels fall, your body treats it as an emergency. The first response is releasing glucagon from the pancreas, a hormone that tells your liver to dump its stored glucose into the bloodstream. If that isn’t enough, your adrenal glands release adrenaline and cortisol, and your pituitary gland releases growth hormone. All of these work together to push glucose levels back up.
That surge of adrenaline is why low blood sugar feels the way it does. Many of the symptoms people notice first, like a racing heart, sweating, shakiness, and sudden anxiety, are actually side effects of the adrenaline response, not the low glucose itself. Your body is essentially triggering a mild fight-or-flight reaction to force glucose out of storage.
Sometimes this hormonal rescue system works well enough that your glucose rebounds on its own. Other times, especially if you have diabetes or an underlying condition, these hormones can’t raise levels fast enough and symptoms get worse.
Early Symptoms vs. Dangerous Symptoms
Low blood sugar produces two distinct waves of symptoms. The first wave comes from your body’s adrenaline response and serves as a warning signal:
- Shakiness or trembling
- Sweating
- Fast or irregular heartbeat
- Sudden hunger or nausea
- Irritability or anxiety
- Tingling in your lips, tongue, or cheeks
If glucose continues dropping, a second wave hits. These symptoms come from your brain not getting enough fuel:
- Trouble concentrating or confusion
- Slurred speech
- Blurry or tunnel vision
- Clumsiness or poor coordination
- Drowsiness
- Seizures or loss of consciousness (in severe cases)
The first set of symptoms is uncomfortable but manageable. The second set means your brain is running out of fuel and you need to act immediately. If someone with low blood sugar becomes confused or loses consciousness, they need emergency help.
Common Causes if You Have Diabetes
For people managing diabetes, low glucose episodes are a well-known side effect of treatment. Insulin and certain oral diabetes medications lower blood sugar by design, and sometimes they overshoot. This can happen when you take your usual dose but eat less than expected, exercise more intensely than planned, or drink alcohol, which impairs your liver’s ability to release stored glucose.
Timing matters too. Some insulin formulations peak hours after injection, which means a dose taken at dinner could drop your glucose in the middle of the night. Nocturnal hypoglycemia is particularly common in people taking a type of insulin that becomes most active six to eight hours after each dose. Signs that your blood sugar dropped overnight include waking up sweaty, having nightmares, or feeling unusually tired and irritable in the morning.
Common Causes if You Don’t Have Diabetes
Low glucose without diabetes is less common but has several possible explanations. One of the most frequent is reactive hypoglycemia, where your blood sugar drops within four hours after eating. Your body releases too much insulin in response to a meal, overshooting what’s needed and sending glucose below normal. This often happens after meals heavy in refined carbohydrates. The exact reason some people’s insulin response overshoots isn’t always clear.
Other causes in people without diabetes include:
- Medications: Accidentally taking someone else’s diabetes medication is one possibility. Other drugs, particularly in people with kidney problems, can lower blood sugar as a side effect.
- Organ disease: Severe liver illness (like cirrhosis or hepatitis), advanced kidney disease, serious infections, and advanced heart disease can all impair your body’s ability to maintain glucose levels.
- Hormone deficiencies: Problems with your adrenal glands or pituitary gland can reduce the hormones your body needs to keep glucose stable. In children, too little growth hormone can cause low blood sugar.
- Alcohol: Drinking, especially on an empty stomach, blocks your liver from releasing glucose.
- Prior stomach surgery: Gastric bypass and other bariatric procedures can change how quickly food moves through your digestive system, sometimes triggering reactive lows.
- Rare tumors: Certain tumors, particularly those in the pancreas, can produce excess insulin.
Why Some People Stop Feeling the Warning Signs
One of the more concerning aspects of repeated low blood sugar is that your body can gradually stop sounding the alarm. This is called hypoglycemia unawareness. When someone experiences frequent episodes of low glucose, the threshold that triggers warning symptoms keeps dropping. If you used to feel shaky at 60 mg/dL, after repeated lows you might not notice anything until you’re at 55 or even lower.
The problem is that the glucose level causing unconsciousness doesn’t drop along with it. So the gap between “I feel fine” and “I’m passing out” shrinks. This puts people at risk for sudden loss of consciousness while driving, working, or sleeping. Recurrent severe episodes have also been linked to a higher risk of heart attack and stroke in the following year, along with potential long-term effects on brain function.
What to Do When Your Glucose Drops
The standard approach is called the 15-15 rule: eat or drink 15 grams of fast-acting carbohydrates, then wait 15 minutes and check your blood sugar again. Fifteen grams of carbs looks like about four glucose tablets, half a cup of juice or regular soda, or a tablespoon of honey. If your glucose is still low after 15 minutes, repeat. Once it comes back up, eat a small meal or snack with some protein and complex carbs to keep it stable.
If someone is too confused or unconscious to eat safely, don’t try to put food in their mouth. This is a medical emergency.
How Recurring Lows Are Investigated
A single mild episode, especially one tied to a skipped meal or unusual exertion, doesn’t necessarily point to an underlying problem. But if your glucose drops repeatedly without an obvious reason, doctors typically want to figure out why.
The most definitive test is a supervised fast that can last up to 72 hours, conducted in a medical setting. You stop eating while your blood sugar and insulin levels are monitored at regular intervals. The goal is to see whether your body is producing insulin when it shouldn’t be, which would suggest a problem like a pancreatic tumor. If reactive hypoglycemia is suspected instead, a five-hour glucose tolerance test, where you drink a sugary solution and your glucose and insulin are tracked over time, is more useful. Doctors may also check cortisol levels or test adrenal function if a hormone deficiency seems likely.
For people with diabetes who are experiencing frequent lows, the investigation is usually more straightforward: reviewing medication doses, meal timing, and exercise patterns to find the mismatch causing the drops. A continuous glucose monitor can reveal patterns, especially overnight dips, that finger-stick checks might miss.