Insulin is a hormone produced by your pancreas that lowers blood sugar by helping cells absorb glucose from the bloodstream. It is the only hormone in the body capable of reducing blood glucose concentration, which makes it essential to survival. When insulin production fails or the body stops responding to it properly, blood sugar rises and diabetes develops.
How Insulin Works in the Body
Insulin is made by specialized cells called beta cells, clustered in small groups within the pancreas known as islets. When you eat and your blood sugar rises, beta cells detect the change and release insulin into the bloodstream. The hormone then travels to cells throughout the body, particularly in skeletal muscle, which handles the majority of insulin-driven glucose uptake.
At the cell surface, insulin binds to a receptor and triggers a chain of internal signals. The end result is that glucose transporters, which normally sit inside the cell, move to the outer membrane and act like gates, allowing glucose to flow in. Without insulin, those gates stay closed and glucose accumulates in the blood instead of fueling your cells.
More Than Just Blood Sugar
Lowering blood sugar is insulin’s most well-known job, but the hormone manages several other metabolic processes simultaneously. In the liver, insulin slows the production of new glucose and promotes glycogen synthesis, essentially telling the liver to store sugar for later rather than dumping it into the bloodstream. This is one reason blood sugar drops after insulin is released.
Insulin also plays a central role in fat storage. It stimulates fat production, increases the activity of enzymes that build fatty acids, and blocks the breakdown of existing fat. At the same time, insulin promotes protein synthesis and prevents protein breakdown. In short, insulin is a storage hormone: when levels are high, the body shifts into “store and build” mode rather than “break down and burn.”
What Happens When Insulin Fails
Diabetes develops through two fundamentally different paths, both involving insulin.
In type 1 diabetes, the immune system destroys the beta cells that produce insulin. Without any meaningful insulin production, blood sugar rises dangerously, and the person requires insulin injections to survive. This form typically appears in childhood or adolescence, though it can develop at any age.
Type 2 diabetes is more complex. It involves a mix of insulin resistance, where cells stop responding efficiently to insulin, and a gradual decline in insulin production. Early on, the pancreas compensates by making more insulin, but over time beta cell mass decreases by roughly 20 to 40 percent in people with long-standing type 2 diabetes. The ratio of resistance to deficiency varies from person to person and can shift as the disease progresses. Sustained high blood sugar itself further damages both insulin secretion and sensitivity, creating a cycle that worsens over time.
Normal Blood Sugar Ranges
A healthy fasting blood sugar, measured after at least eight hours without food, falls below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes, a stage where insulin is already struggling to keep up. A fasting level of 126 mg/dL or higher on two separate tests meets the diagnostic threshold for diabetes.
These numbers reflect how well your body’s insulin system is functioning. If your doctor wants a more direct measurement of insulin production, they may order a C-peptide test. C-peptide is released alongside insulin in equal amounts, so its level in the blood reveals how much insulin the pancreas is actually making. A very low stimulated C-peptide (at or below 0.2 nmol/L) signals severe beta cell loss and a likely need for insulin therapy.
Types of Insulin Medication
For people whose bodies no longer make enough insulin, synthetic versions replace what’s missing. These come in several categories, designed to mimic the body’s natural insulin patterns.
- Rapid-acting: Starts working in about 15 minutes, peaks within one to two hours, and lasts two to five hours. Taken just before meals to handle the blood sugar spike from food.
- Short-acting (regular): Kicks in within 30 to 60 minutes, peaks between two and four hours, and lasts five to eight hours. Also used around mealtimes but requires earlier timing.
- Medium-acting (NPH): Begins working in two to four hours, peaks between four and ten hours, and lasts eight to sixteen hours. Often used as background insulin but requires careful timing because of its pronounced peak.
- Long-acting: Takes two to four hours to start, has little or no peak, and provides steady coverage for about 24 hours. Taken once daily to maintain a baseline insulin level.
- Ultra-long-acting: Provides coverage for up to 42 hours with no peak, offering the most stable background insulin. Some people find this reduces overnight blood sugar swings.
Many people with type 1 diabetes use a combination of long-acting insulin for baseline coverage and rapid-acting insulin at meals. People with type 2 diabetes may start with long-acting insulin alone if oral medications are no longer enough. A once-weekly insulin formulation has recently been approved in the EU, Canada, Japan, Australia, and several other countries for both type 1 and type 2 diabetes, which could reduce the number of injections from 365 per year to 52.
Storing Insulin Properly
Insulin is a protein, and like most proteins, it breaks down when exposed to extreme temperatures. Unopened insulin should be kept refrigerated between 36°F and 46°F, where it stays potent until the expiration date printed on the package. Once you open a vial or pen, it can be kept at room temperature (59°F to 86°F) for up to 28 days. After that, it should be discarded even if insulin remains. Never freeze insulin, and avoid leaving it in a hot car or in direct sunlight. If insulin has been diluted or transferred out of its original container, the window shortens to two weeks.
Cost and Access
Insulin affordability has been a major concern in the United States. More than 30 states plus Washington, D.C. now have copay cap laws that limit what insured patients pay out of pocket for a 30-day supply. These caps range from $0 in New York to $100 in states like Colorado, Delaware, and Vermont. Many states, including Illinois, Maine, Montana, Nevada, New Jersey, Oregon, and Washington, have set the cap at $35 per month. Connecticut, Massachusetts, New Mexico, North Dakota, and Texas cap costs at $25.
These caps apply to state-regulated insurance plans. If you’re on a large employer plan governed by federal law, a separate federal cap of $35 per month applies to Medicare enrollees, and some large group plans have adopted similar limits. Coverage varies, so checking with your insurer or your state’s insurance department can clarify what you’ll actually pay.