There is no single “best” insulin for type 2 diabetes. The right choice depends on where you are in your treatment journey, how your blood sugar behaves throughout the day, your tolerance for injections, and your risk of side effects like low blood sugar and weight gain. That said, most people with type 2 diabetes start with a long-acting basal insulin, and the evidence shows that the major options lower blood sugar equally well. The differences come down to side effect profiles, dosing convenience, and cost.
Why Insulin Becomes Necessary
Type 2 diabetes is progressive. Over time, the cells in your pancreas that produce insulin gradually lose function, and oral medications or other injectables may no longer keep your blood sugar in range. Current guidelines from the American Diabetes Association say insulin can be considered at any stage of type 2 diabetes, regardless of what other medications you’re taking, when the situation calls for it. That might mean your A1c remains stubbornly high, you’re experiencing symptoms of very high blood sugar, or you’ve been newly diagnosed with levels so elevated that insulin is the fastest way to bring them down.
How Insulin Types Differ
Insulins are grouped by how quickly they start working and how long their effects last. Understanding these categories helps you see why your doctor might recommend one type over another.
- Long-acting (basal) insulin: Starts working in about 2 hours, has no sharp peak, and lasts up to 24 hours. This is the workhorse for most people with type 2 diabetes because it provides a steady background level of insulin all day.
- Intermediate-acting insulin: Takes 2 to 4 hours to kick in, peaks between 4 and 12 hours, and lasts 12 to 18 hours. Less commonly used now that long-acting options are available, but still prescribed in some regimens.
- Rapid-acting (bolus) insulin: Starts in about 15 minutes, peaks at 1 hour, and wears off in 2 to 4 hours. Used before meals to cover the blood sugar spike from eating.
- Short-acting (regular) insulin: Takes 30 minutes to begin working, peaks at 2 to 3 hours, and lasts 3 to 6 hours. An older option for mealtime coverage.
Basal Insulin: The Usual Starting Point
For most people with type 2 diabetes, treatment begins with a single daily injection of basal insulin, often added to metformin or other oral medications. The three main basal insulins are glargine, detemir, and degludec. A large meta-analysis covering 70 studies found no statistically significant difference in A1c reduction between any of them. They all lower blood sugar about equally well.
Where they diverge is in side effects. Degludec (an ultra-long-acting insulin lasting beyond 24 hours) was associated with roughly 27% fewer episodes of low blood sugar compared to glargine in people with type 2 diabetes, and about 28% fewer episodes of dangerous severe low blood sugar. For anyone who has experienced hypoglycemia or is at higher risk for it, that’s a meaningful advantage.
Detemir tends to cause slightly less weight gain, about 1 kilogram less than glargine over a study period. However, it also had more than double the rate of people stopping treatment due to side effects compared to glargine. So the modest weight benefit comes with a trade-off in tolerability for some people.
In practice, glargine remains the most widely prescribed basal insulin, partly because of familiarity and partly because interchangeable biosimilar versions have brought the cost down. Degludec is often considered when low blood sugar is a particular concern or when a more flexible dosing schedule would help.
Weight Gain on Insulin
Weight gain is one of the most common concerns people have about starting insulin, and it’s a real effect. In the first year of insulin therapy, body weight typically increases by about 2 to 3 kilograms (roughly 5 to 7 pounds). The UK Prospective Diabetes Study, one of the largest trials in type 2 diabetes, found an average gain of 3.1 kilograms over 10 years in the group treated more intensively with insulin.
Several things drive this. Insulin is an anabolic hormone, meaning it promotes fat and muscle storage. Before starting insulin, very high blood sugar causes your body to lose calories through your urine as excess glucose spills over. Once insulin corrects this, your body retains those calories again. There’s also a behavioral component: episodes of low blood sugar lead to “defensive snacking,” and some people develop a false sense of dietary freedom because insulin is managing their numbers. Notably, one study found that weight continued to climb even after blood sugar levels had stabilized, suggesting insulin’s growth-promoting effects go beyond glucose control alone.
When Mealtime Insulin Gets Added
If your A1c and post-meal blood sugar readings remain above target despite increasing your basal insulin dose, the next step is often adding rapid-acting insulin before meals. A general sign that you’ve reached this point is needing more than about 0.5 units of basal insulin per kilogram of body weight per day without adequate control. At that dose, you’re likely “over-basalized,” meaning more background insulin won’t solve the problem because the real issue is blood sugar spikes after eating.
Adding mealtime insulin doesn’t necessarily mean three new injections per day right away. Many providers start with the “basal plus” approach: one injection of rapid-acting insulin before your largest meal. A typical starting dose is around 4 to 6 units, or roughly 0.1 units per kilogram of body weight. For someone weighing 110 kilograms (about 240 pounds), that would be about 11 units before their biggest meal. From there, doses are adjusted based on how your post-meal numbers respond. A second mealtime injection can be added later if needed.
The overall goal in a full basal-bolus regimen is a roughly 50/50 split between your total daily basal dose and your total daily mealtime dose.
Combination Insulin and GLP-1 Products
One of the more significant advances in insulin therapy combines basal insulin with a GLP-1 receptor agonist (the same class of drug as semaglutide and liraglutide) in a single injection. Two products are currently available: one pairs degludec with liraglutide, and the other pairs glargine with lixisenatide.
The appeal of these combinations is that the GLP-1 component counteracts two of insulin’s biggest downsides. In clinical trials, both products lowered A1c more than basal insulin alone, with no increase in weight gain and a similar or lower rate of low blood sugar episodes. The GLP-1 component helps blunt post-meal blood sugar spikes, something basal insulin alone struggles with, while also reducing appetite. For people who need insulin but are concerned about weight or want fewer total injections than a basal-bolus regimen would require, these fixed-ratio combinations are worth discussing.
Inhaled Insulin
Afrezza is an inhaled rapid-acting insulin approved for mealtime use in adults with type 2 diabetes. It reaches peak levels in the bloodstream in about 15 minutes, considerably faster than injected rapid-acting insulin (which takes closer to 2 hours to peak). That speed more closely mimics how a healthy pancreas responds to food.
The trade-offs are notable. Because your lungs absorb insulin less efficiently than an injection site, you need to inhale roughly four times as much insulin to get the same effect. A1c reductions in clinical trials were about 7% less than those achieved with injected insulin. On the positive side, hypoglycemia rates were lower (about 12% of patients versus 18% with injected insulin), and the risk of delayed low blood sugar episodes appears reduced because the insulin clears your system faster. Afrezza is only approved for non-smokers without lung disease, and it cannot replace basal insulin. It covers meals only.
Biosimilars and Cost
Cost used to be one of the biggest barriers to insulin therapy. That has changed significantly. The Inflation Reduction Act capped insulin copays at $35 per month for people on Medicare, and eight states have enacted their own caps ranging from $25 to $100 for a 30-day supply. A nonprofit generic drug company is also partnering with state programs to sell generic insulin vials for no more than $30 and pen cartridges for no more than $55 per box of five.
On the biosimilar front, the FDA has approved an interchangeable biosimilar version of glargine. “Interchangeable” means your pharmacist can substitute it for the brand-name product without needing a new prescription from your doctor, just as they would with a generic pill. This has made glargine, already the most commonly prescribed basal insulin, one of the most affordable options as well.
Once-Weekly Insulin on the Horizon
A once-weekly basal insulin called icodec has been tested in a 26-week clinical trial against daily glargine in people with type 2 diabetes who hadn’t previously used insulin. The results showed similar blood sugar lowering and a comparable safety profile. One injection per week instead of seven could make a real difference in adherence and quality of life. This product is in late-stage development and is not yet widely available, but it signals a shift toward less frequent dosing in the near future.