Insulin Isophane (NPH)

Isophane insulin, almost always called NPH insulin after its full name Neutral Protamine Hagedorn, is an intermediate-acting insulin preparation used as a basal, or background, insulin ^[2]. It consists of regular human insulin combined with the protein protamine and a small amount of zinc, an arrangement that forms crystals and delays absorption ^[2]. Unlike the clear solutions of regular insulin and the modern analogs, NPH is a cloudy suspension that must be gently rolled or tipped to redistribute the crystals before each injection ^[1].

NPH insulin grew out of work by the Danish physician Hans Christian Hagedorn, who with colleagues found in the 1930s that adding protamine to insulin prolonged its action ^[3]. The neutral, protamine-based crystalline formulation known as NPH was introduced in 1946 and became one of the earliest deliberately engineered forms of slow-release drug delivery ^[2]. For decades it was the standard intermediate and basal insulin, and it remains widely used, though long-acting analogs have taken over much of this role ^[4]. Originally derived from animal pancreases, the insulin in modern NPH is human insulin made by recombinant DNA technology ^[2].

NPH is used to provide baseline insulin coverage in type 1 and type 2 diabetes, injected once or twice a day, often in combination with a mealtime insulin ^[2]. It is also a component of premixed, or biphasic, insulins, in which it is blended in fixed proportion with a short-acting or rapid-acting insulin ^[2]. Its main limitation compared with newer basal insulins is its pronounced peak and day-to-day variability in absorption, which raise the risk of hypoglycemia; comparative trials such as the Treat-to-Target study showed that basal analogs can achieve similar blood-sugar control with fewer nighttime low-glucose episodes ^[4]. Even so, NPH remains valued for its long record of use and lower cost ^[4].

NPH insulin is supplied in vials and pen cartridges under names such as Humulin N, Novolin N, and Insulatard, and appears on the World Health Organization list of essential medicines ^[2]. Like all insulins, its most important risk is hypoglycemia, which with NPH is most likely during its mid-action peak and overnight ^[4]. Other possible effects include low blood potassium, injection-site reactions, thickening or pitting of the fat under the skin at overused sites, weight gain, and, rarely, allergic reactions ^[2].

NPH is prescription only. Its main risk is hypoglycemia, which is most likely around the 4 to 8 hour peak and overnight, so meal timing and bedtime snacks matter. Other effects include weight gain, injection site lipohypertrophy and, at high doses, low potassium. Its pronounced peak raises the chance of nighttime lows compared with flatter analog insulins like glargine. It must be gently resuspended by rolling before each dose, and the dose needs adjusting for illness, activity and other glucose-lowering drugs.

NPH insulin was created by the Danish researcher Hans Christian Hagedorn and his colleagues at the Nordisk Insulinlaboratorium in the 1930s and 1940s, and the name Neutral Protamine Hagedorn preserves his contribution. Hagedorn had earlier discovered that adding protamine, a small protein obtained from fish sperm, prolonged insulin's action; the key refinement was combining insulin, protamine, and zinc in precisely matched proportions so that the components crystallized together, an arrangement termed isophane.

This produced a stable, intermediate-acting suspension that could be injected once or twice daily to provide background insulin coverage, and it was introduced clinically in 1950. For much of the second half of the twentieth century NPH was the dominant basal insulin worldwide, often mixed with regular insulin in premixed formulations. Although long-acting analogs have since taken much of its market, NPH remains widely used and is listed by the World Health Organization as an essential medicine.

NPH insulin is respected as the affordable, time-tested basal insulin that provided background glucose control for generations of people with diabetes long before designer analogs existed. Its low cost keeps it clinically important, especially in resource-limited settings and for patients paying out of pocket, and it remains a reasonable choice when a pronounced peak can be used strategically, such as covering an overnight or midday rise.

Decades of use have made its behavior thoroughly familiar to clinicians. In candid comparison, however, its distinct mid-action peak and day-to-day variability give it a higher tendency to cause hypoglycemia, particularly at night, than the flatter modern analogs; a landmark trial showed insulin glargine produced significantly less nocturnal hypoglycemia at similar control. It also requires gentle remixing before each dose to keep the cloudy suspension uniform, a small but real practical demand.