Insulins are usually grouped as fast-acting, intermediate-acting, or long-acting. Within these groups, they can be further classified as human insulin and human insulin analogs. An insulin analog is a human insulin that has one or two amino acids changed which affects how quickly it is absorbed after injection and how fast or slow it acts. Insulin analogs are usually given within 15 minutes of a meal or at the same time as food.

What are Fast-Acting Insulins?

Fast-acting insulins are absorbed quickly into the bloodstream and are used to control blood sugar levels during meals or snacks, to correct unexpectedly high blood sugar levels, or as the main insulin used in an insulin pump. Fast-acting insulins can be further divided into rapid-acting insulin analogs or short-acting human insulin (also referred to as regular insulin).

What are rapid-acting insulin analogs?

These work within 5 to 15 minutes and peak within 1 to 2 hours. Larger dosages of insulin analogs affect the duration of action, for example, a few units may last less than 4 hours, whereas 25 to 30 units may last 5 to 6 hours. However, a larger dose does not affect the onset of action or the time it takes to peak.

• Examples include insulin lispro (Humalog, Admelog, Lyumjev), insulin aspart (Fiasp, Novolog), and insulin glulisine (Apidra).

What are short-acting insulins/regular human insulins?

These work within half to one hour and peak within two to four hours. Larger doses of human insulin result in a faster onset of action, but a delayed peak effect and a longer duration of action.

• Examples of regular insulin include Humulin R and Novolin R.

What are intermediate-acting insulins?

Although you can still buy intermediate-acting human insulins, their use has largely been superseded by human insulin analogs.

• Intermediate-acting insulins may also be called isophane insulin or NPH (stands for Neutral Protamine Hagedorn) insulin. Examples include Humulin N and Novolin N.

Intermediate-acting insulins are absorbed more slowly and last for longer than short-acting insulins. They typically start to work within 1 to 2 hours and peak within four to six hours. Their duration of action can be anything from 12 to 24 hours and depends on the actual dose (higher dosages have a prolonged duration of effect and take longer to peak).

They cover elevations in blood glucose that occur when fast-acting insulins stop working. They are typically used to control blood sugar levels overnight or to mimic the natural basal release of insulin that occurs during fasting or between meals. Intermediate-acting insulins are usually administered twice a day but may be given as a single dose overnight. They may be premixed with short-acting human insulin (for example, Humulin 70/30 or Novolin 70/30).

Premixes combining rapid-acting and intermediate-acting insulin analogs are also available, examples include Humalog mix 75/25 and Novolog 70/30.

What are long-acting and ultra-long-acting insulins?

Long-acting insulin analogs release insulin as a steady amount over most of the day, without any noticeable peak in insulin levels, mimicking natural basal insulin release.

They are preferred over intermediate-acting insulins (NPH insulin) because they have a longer duration of action, flatter concentration profile, more consistent effects, and less risk of hypoglycemia (low blood sugar levels).

• Long-acting insulin analogs that are currently available are insulin detemir (Levemir) and insulin glargine (Lantus, Semglee, Toujeo Solostar), and insulin glargine-aglr (Rezvoglar Kwikpen), a biosimilar to Lantus that is interchangeable with it.

Detemir starts to work in about an hour and it reaches a plateau in five hours. The effects of glargine usually last for 24 hours. Both types are considered safe and effective; however, there may be less interindividual variability, risk of hypoglycemia, or weight gain with glargine.

• Insulin degludec (Tresiba) is an ultra-long-acting insulin that can last in the body for up to 42 hours, although it is generally still administered once daily. Another advantage is that once the pen is in use, it is good for 8 weeks either refrigerated or at room temperature (below 86°F).

What are concentrated insulins?

Most insulin in the U.S. is sold at a concentration of 100 units per ml (U100). However, there are five concentrated products on the market:

• Humulin R (human insulin) which has a 500 units per ml (U500) strength available
• Humalog (insulin lispro) which has a 200 units per ml (U200) strength available
• Lyumjev (insulin lispro-aabc) which has 200 units per ml (U200) strength available in addition to the U100 strength
• Toujeo Max Solostar which contains 300 units per ml (U300) of insulin glargine
• Tresiba (insulin degludec) which has a 200 units per ml (U200) strength available in addition to the U100 strength.