NPH insulin is an intermediate-acting insulin used to manage blood sugar in people with type 1 and type 2 diabetes. Its name stands for Neutral Protamine Hagedorn, a reference to the protamine protein and neutral pH used in its formulation. NPH has been available for decades, remains one of the most affordable insulin options on the market, and is still widely prescribed today.
How NPH Insulin Works
NPH insulin is made by combining lab-produced human insulin with a protein called protamine and a small amount of zinc. This creates tiny insulin crystals that are suspended in liquid, giving NPH its characteristic cloudy appearance. When you inject it under the skin, those crystals pile up at the injection site and dissolve gradually. Your body’s own enzymes and immune cells slowly break apart the crystal clusters, releasing insulin into the bloodstream over several hours.
This slow-dissolve design is what makes NPH “intermediate-acting.” It doesn’t hit your bloodstream all at once like rapid-acting insulin does at mealtimes. Instead, it provides a background level of insulin that helps control blood sugar between meals and overnight. NPH typically begins working within 1 to 2 hours after injection, reaches its strongest effect around 4 to 8 hours in, and continues working for roughly 12 to 16 hours total.
The Peak Effect and Why It Matters
One of the most important things to understand about NPH is that it has a pronounced peak. Unlike newer long-acting insulins that release at a relatively steady rate over 24 hours, NPH delivers a noticeable surge of insulin roughly midway through its action window. This peak can be useful if it lines up with a meal, but it also means you need to eat on a fairly consistent schedule. If you skip a meal or eat later than planned during that peak window, your blood sugar can drop too low.
Many people take NPH twice a day, once in the morning and once at bedtime, to approximate around-the-clock background insulin coverage. Because the peak from a bedtime dose tends to hit in the middle of the night, nighttime low blood sugar is a well-documented concern. In a large meta-analysis published in Diabetes Care comparing NPH to a newer long-acting insulin, about 38% of people on NPH experienced at least one documented episode of nighttime low blood sugar, compared to 28% on the long-acting alternative. Severe nighttime episodes were uncommon overall (under 2%), but still roughly twice as frequent with NPH. A bedtime snack is a common strategy to help buffer against this overnight dip.
How to Prepare and Inject NPH
Because NPH is a suspension of crystals rather than a clear solution, it separates when it sits still. Before each injection, you need to resuspend the insulin so the dose is consistent. The FDA-approved technique is to roll the vial gently between your hands in a horizontal position, about 10 times, until the liquid looks uniformly white and cloudy. Don’t shake it. Shaking creates air bubbles or froth that can cause you to draw up the wrong amount.
If you’re combining NPH with regular (short-acting) insulin in the same syringe, there’s a specific order to follow: draw the regular insulin first, then the NPH. The easy way to remember this is “clear before cloudy,” since regular insulin is a clear liquid and NPH is cloudy. Drawing in the wrong order can contaminate your regular insulin vial with protamine, which would alter how it works.
Storage Guidelines
Unopened NPH vials and pens should be stored in the refrigerator. Once you start using a vial or pen, it can be kept at room temperature (between 59°F and 86°F) for up to 28 days and still work properly. After 28 days at room temperature, any remaining insulin should be discarded. Cold insulin can sting more on injection, so many people prefer to keep their in-use vial out of the fridge. Just avoid leaving it in a hot car or direct sunlight.
NPH vs. Newer Long-Acting Insulins
NPH was the standard background insulin for decades before long-acting analogs arrived in the early 2000s. Those newer options were designed to release more evenly over a full 24 hours, eliminating the sharp peak that NPH produces. The practical advantages are real: fewer episodes of low blood sugar (particularly at night), more flexibility in meal timing, and once-daily dosing instead of twice daily.
So why does NPH remain widely used? Cost is the biggest reason. Newer long-acting insulins can cost several hundred dollars per month without insurance. NPH, by contrast, is available at Walmart for $25 per vial or $44 for a box of five pens under the Novolin N brand. In most states, you don’t even need a prescription to buy it. For people who are uninsured, underinsured, or facing gaps in coverage, NPH can be the difference between having insulin and going without.
NPH also works well for many people when used correctly. The key is consistent meal timing and careful attention to that peak window. If your daily schedule is predictable and you’re comfortable managing around the peak, NPH can control blood sugar effectively at a fraction of the cost of newer alternatives.
Common Side Effects
The most significant side effect of NPH is hypoglycemia, or low blood sugar. This is more likely if you miss a meal, exercise more than usual, or take too much insulin. Symptoms include shakiness, sweating, confusion, rapid heartbeat, and hunger. The nighttime risk is particularly worth watching for: waking up with a headache, damp sheets from sweating, or unusually high morning blood sugar (a rebound effect) can all signal that your blood sugar dropped too low overnight.
Injection site reactions are also possible. Some people notice small lumps or changes in the fat tissue under the skin, especially if they inject in the same spot repeatedly. Rotating your injection sites helps prevent this. Weight gain is another common effect of insulin therapy in general, not unique to NPH, because insulin helps your body store glucose more efficiently.
Who Uses NPH Insulin
NPH is used by people with both type 1 and type 2 diabetes. In type 2 diabetes, it’s often the first injectable insulin added when oral medications alone aren’t keeping blood sugar in range. In type 1 diabetes, it’s typically paired with a rapid-acting insulin taken at mealtimes to mimic the body’s natural two-part insulin pattern: a steady background level plus bursts at meals.
NPH is also commonly used during pregnancy for gestational diabetes, partly because it has a long safety track record and partly because some newer insulin analogs have less extensive data in pregnant populations. Its affordability makes it a mainstay in lower-income countries and community health settings worldwide.