Blood sugar drops when your body clears glucose from the bloodstream faster than it can replace it. This can happen for a surprising number of reasons, from skipping a meal to drinking alcohol to producing too much insulin after eating. A reading below 70 mg/dL is considered low, and below 54 mg/dL is considered severely low and requires immediate action.
Understanding why this happens starts with how your body normally keeps blood sugar stable, and what can throw that system off balance.
How Your Body Regulates Blood Sugar
Your pancreas produces two hormones that work like a thermostat for blood sugar. After you eat, rising glucose triggers the release of insulin, which drives sugar into your muscle, fat, and liver cells for storage. Between meals and overnight, insulin levels fall and a second hormone, glucagon, signals your liver to break down its stored glucose (called glycogen) and release it back into the bloodstream. Your liver can also build new glucose from scratch using other raw materials.
This back-and-forth between insulin and glucagon keeps your blood sugar in a narrow range all day long. Problems arise when something disrupts the balance: too much insulin, not enough glucagon, or a liver that can’t release glucose when it should.
Blood Sugar Drops After Eating
One of the more confusing patterns is blood sugar falling within one to four hours after a meal. This is called reactive hypoglycemia, and it happens when your body releases more insulin than the meal actually required. The excess insulin clears too much glucose out of your blood, leaving you shaky, lightheaded, or foggy-headed shortly after eating.
In people without diabetes, the exact cause often isn’t clear, but it tends to be connected to what and when you eat. Meals heavy in refined carbohydrates can spike blood sugar fast, prompting an oversized insulin response that overshoots. People who have had gastric bypass or other bariatric surgery are especially prone to this, because food moves into the small intestine more quickly, triggering a rapid surge of insulin.
Other, rarer causes of reactive hypoglycemia include inherited metabolic conditions and certain types of tumors that affect insulin production.
Diabetes Medications
For people with diabetes, medications are the most common reason blood sugar drops too low. Insulin injections and a class of oral medications called sulfonylureas are the main culprits. Sulfonylureas work by forcing the pancreas to release preformed insulin regardless of what your blood sugar is doing at that moment, and they also suppress glucagon. That combination can push glucose well below normal, especially if you take the medication and then eat less than expected, skip a meal, or exercise.
Not all diabetes drugs carry this risk. Most newer oral medications and injectable therapies don’t push blood sugar below normal even in large doses. They’re technically “antihyperglycemic,” meaning they lower high blood sugar without the same danger of overshooting into low territory. If you experience frequent lows, the type of medication you’re on matters a lot.
Alcohol and the Liver
Drinking alcohol, particularly on an empty stomach, can cause blood sugar to drop for hours. The reason is that alcohol directly interferes with your liver’s ability to produce new glucose. Normally, when blood sugar starts falling between meals, your liver ramps up glucose production. Ethanol disrupts this process at a chemical level, blocking the signals that tell liver cells to convert stored fuel into glucose. The result is that your liver essentially goes offline as a glucose source right when you need it most.
This effect is strongest during fasting, so drinking without eating is particularly risky. The combination of alcohol and diabetes medication compounds the danger further, because the medication is lowering blood sugar from one direction while alcohol removes the safety net from the other.
Exercise and Physical Activity
Working muscles pull glucose out of the bloodstream at a much higher rate than resting muscles. That’s why exercise lowers blood sugar, and the effect doesn’t end when you stop moving. Physical activity can keep blood sugar lower for up to 24 hours afterward by increasing your body’s sensitivity to insulin. During that window, the same amount of insulin moves more glucose into cells than it normally would.
This post-exercise sensitivity is why blood sugar drops can catch people off guard well after a workout, sometimes overnight following an evening gym session. For people on insulin or sulfonylureas, adjusting food intake or medication timing around exercise is often necessary to avoid lows.
Skipping Meals and Prolonged Fasting
Your liver stores enough glycogen to maintain blood sugar for roughly 12 to 24 hours of fasting. After that, it relies on building new glucose from amino acids and other sources. In most healthy people, this system works well enough that skipping a meal causes no real problem. But if anything else is compromising that backup system (alcohol, certain medications, a hormonal deficiency), going without food becomes the tipping point that sends blood sugar too low.
People who eat irregularly or follow very restrictive diets sometimes experience mild drops that cause symptoms like irritability, shakiness, and difficulty concentrating, even if their blood sugar doesn’t fall below the clinical threshold of 70 mg/dL.
Hormonal Causes
Several hormones besides insulin and glucagon help raise blood sugar when it falls. Cortisol, epinephrine (adrenaline), and growth hormone all signal the body to release stored fuel. If any of these are deficient, your ability to recover from a blood sugar dip is weakened.
Adrenal insufficiency is the most recognized hormonal cause. When the adrenal glands don’t produce enough cortisol, the body struggles to boost blood sugar through its normal counter-regulatory pathways. Cortisol acts on multiple tissues to mobilize glucose, so losing that signal leaves a significant gap. Low levels of growth hormone or epinephrine can contribute in a similar way, though these are less common causes on their own.
What Low Blood Sugar Feels Like
Symptoms generally begin as blood sugar falls below 70 mg/dL, though the exact threshold varies from person to person. The first wave of symptoms comes from your body’s adrenaline response to the drop: shakiness, sweating, a pounding heart, hunger, and anxiety. These are your body’s early warning signals, and they’re designed to get your attention.
If blood sugar continues falling below roughly 54 mg/dL, the brain itself starts running short on fuel. At this stage, symptoms shift to confusion, difficulty speaking, blurred vision, poor coordination, and drowsiness. Severe episodes can cause loss of consciousness and require someone else’s help to treat. The American Diabetes Association classifies these as three escalating levels:
- Level 1: Blood sugar between 54 and 69 mg/dL. You’ll likely notice adrenaline-driven symptoms and can treat it yourself.
- Level 2: Blood sugar below 54 mg/dL. Brain-related symptoms begin, and you need to act immediately.
- Level 3: A severe event where you need another person’s help to recover, regardless of the specific number on a meter.
How Doctors Confirm the Cause
Feeling shaky or lightheaded doesn’t always mean your blood sugar is truly low. Doctors use a set of criteria called Whipple’s triad to confirm a genuine hypoglycemic disorder. All three must be present: symptoms consistent with low blood sugar, a lab-verified low glucose reading taken during those symptoms, and resolution of symptoms once glucose is raised. If all three check out, further testing can investigate the underlying cause, whether it’s an insulin-producing tumor, a hormonal deficiency, or a medication effect.
This distinction matters because many people who feel “hypoglycemic” after meals actually have blood sugar that’s dropping rapidly but staying within normal range. The speed of the drop, not the absolute number, can trigger symptoms. That experience is real but points to a different set of solutions than true hypoglycemia.