Neutral Protamine Hagedorn (NPH) insulin, also known as isophane insulin, is an intermediate-acting insulin given to help control blood sugar levels in people with diabetes. It is used by injection under the skin once to twice a day. Onset of effects is typically in 90 minutes and they last for 24 hours. Versions are available that come premixed with a short-acting insulin, such as regular insulin.

The common side effect is low blood sugar. Other side effects may include pain or skin changes at the sites of injection, low blood potassium, and allergic reactions. Use during pregnancy is relatively safe for the baby. NPH insulin is made by mixing regular insulin and protamine in exact proportions with zinc and phenol such that a neutral-pH is maintained and crystals form. There are human and pig insulin based versions.

Protamine insulin was first created in 1936 and NPH insulin in 1946. It is on the World Health Organization's List of Essential Medicines. NPH is an abbreviation for "neutral protamine Hagedorn". In 2020, insulin isophane was the 221st most commonly prescribed medication in the United States, with more than 2 million prescriptions. In 2020, the combination of human insulin with insulin isophane was the 246th most commonly prescribed medication in the United States, with more than 1 million prescriptions.