Glucagon is a hormone that raises blood sugar. Produced by alpha cells in the pancreas, it works as the direct counterpart to insulin: while insulin lowers blood sugar after you eat, glucagon pulls it back up when levels drop too low. This balancing act keeps your brain, muscles, and organs fueled between meals, during sleep, and anytime you go without food for more than a few hours.
How Glucagon Raises Blood Sugar
Glucagon’s primary target is the liver, which stores glucose in a packed-together form called glycogen. When glucagon arrives at liver cells, it triggers a chain reaction that breaks glycogen apart into individual glucose molecules. An enzyme called glycogen phosphorylase clips glucose units off the glycogen chain one by one, while a separate debranching enzyme handles the spots where the chain forks. The end result is free glucose that the liver releases directly into your bloodstream.
This process is the body’s first line of defense against falling blood sugar, and it kicks in quickly. But glycogen stores are limited. After about 4 to 6 hours of fasting, the liver starts running low and glucagon shifts to a second strategy: building brand-new glucose from scratch. The liver assembles glucose from raw materials like lactate (a byproduct of muscle activity), glycerol (released from stored fat), and certain amino acids from protein breakdown. This backup system, called gluconeogenesis, peaks around 24 hours into a fast, once glycogen reserves are essentially depleted.
What Triggers Glucagon Release
Alpha cells in the pancreas constantly monitor blood sugar and ramp up glucagon secretion as glucose levels fall. The relationship isn’t a simple on/off switch. Neighboring cells in the pancreas help fine-tune the response. As blood sugar rises, beta cells release insulin and delta cells release a suppressive signal called somatostatin, both of which put the brakes on glucagon. When blood sugar drops, those braking signals weaken and glucagon flows freely.
In practical terms, glucagon secretion is already being suppressed at normal blood sugar levels (around 90 mg/dL). It increases progressively as blood sugar dips below that range, with the strongest release happening during significant drops, such as a skipped meal combined with physical activity or too much insulin in someone with diabetes.
Glucagon’s Role Beyond Blood Sugar
Glucagon has historically been linked to fat breakdown, and early lab experiments suggested it could stimulate fat cells to release fatty acids. However, more recent research paints a different picture. Glucagon receptors on fat cells are present in only negligible amounts compared to the liver, and glucagon concentrations in the general bloodstream are much lower than in the portal vein that feeds directly into the liver. Studies measuring free fatty acid levels during glucagon infusion found no significant changes, suggesting that glucagon’s direct effect on fat tissue is minimal under normal conditions. Its fat-related effects appear to happen primarily in the liver, where it promotes the oxidation of fatty acids for energy.
Glucagon as an Emergency Medication
For people with diabetes, severe low blood sugar (hypoglycemia) can cause confusion, seizures, or loss of consciousness. When someone can’t eat or drink to bring their sugar back up, an injection or nasal spray of synthetic glucagon can be lifesaving. The standard dose for adults and children over 25 kg is 1 mg, while smaller children receive 0.5 mg. If there’s no response after 15 minutes, a second dose can be given.
How fast it works depends on how it’s delivered. Given intravenously, glucagon pushes blood sugar to its peak within 5 to 20 minutes. An intramuscular injection, which is more common outside a hospital setting, takes about 30 minutes to reach peak effect. A nasal spray version eliminates the need for needles entirely and is equally effective at resolving low blood sugar episodes, making it easier for family members or bystanders to administer in an emergency.
When the Body Makes Too Much Glucagon
In rare cases, a tumor on the pancreas called a glucagonoma causes the body to overproduce glucagon. Normal fasting glucagon levels sit below 150 pg/mL; glucagonoma patients typically show levels above 500 pg/mL. The excess glucagon keeps blood sugar persistently elevated and often causes a distinctive skin rash (a red, blistering eruption that tends to appear around the groin and legs), unexplained weight loss, and anemia. These tumors are uncommon, but the combination of a stubborn rash with rising blood sugar is a hallmark pattern that prompts doctors to check glucagon levels.
How Glucagon and Insulin Work Together
Thinking of glucagon in isolation misses the bigger picture. Blood sugar stability depends on the ratio between glucagon and insulin, not the absolute level of either one. After a carbohydrate-rich meal, insulin surges and glucagon dips, directing glucose into cells for energy or storage. During an overnight fast, the pattern reverses: insulin drops, glucagon rises, and the liver starts exporting glucose to keep your blood sugar from falling dangerously low.
In type 1 diabetes, this partnership breaks down on both sides. The immune system destroys insulin-producing beta cells, but alpha cell function also becomes dysregulated, often releasing too much glucagon after meals and too little during hypoglycemia. This dual defect is one reason blood sugar management in type 1 diabetes can be so challenging, even with careful insulin dosing. In type 2 diabetes, alpha cells often oversecrete glucagon as well, contributing to the higher-than-normal blood sugar levels that define the condition.