How long does insulin work after injection?

Dr. Jack Merendino, MD
Endocrinology Diabetes & Metabolism
Insulin is usually classified as short- or fast-acting, intermediate-acting and long-acting. Short-acting insulin typically begins to act within 30 to 45 minutes, reaches a peak of action at around 90 minutes and has mostly stopped working by about 3 to 4 hours. Short-acting insulin includes a generic form called “regular” insulin and the brand-name insulins Humalog, NovoLog and Apidra. There is also a short-acting form of insulin that is inhaled rather than injected. This was introduced in 2015 under the brand name Afrezza. Short-acting insulin is used to treat the rapid rise and fall of sugar with meals.  

Intermediate-acting insulin, most commonly a type known as “NPH,” typically starts to work somewhat more slowly, reaches a peak of action within about 6 hours and lasts between 12 and 18 hours in most people, though there is lots of variability from one patient to another. This kind of insulin is not used as commonly now as it had been but still works well in some settings.  

Long-acting insulin is meant to provide a background of insulin throughout a 24-hour period. A long-acting insulin, glargine, comes as brand-names Lantus and Toujeo. A generic form will likely become available in 2015 or 2016. Another form of long acting insulin called detemir insulin comes as brand-name Levemir.  

People with type 1 diabetes must either taking short-acting insulin round-the-clock in an insulin pump or they can take a long-acting form of insulin as a background and inject additional fast-acting insulin with meals.  

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