Insulin is classified according to its duration of action, with some preparations designed to be quick-acting while others exert their effects over a longer period of time. In general, among the rapid-acting types, human insulin is the fastest species to take effect, followed by porcine and then bovine.
Very Quick-acting Insulin Analogues
These are newly developed, clear insulins which are the most rapidly absorbed of all the types that are available and the quickest to reach a peak of activity (effect). They begin to act 10 to 20 minutes after being injected and reach a peak of activity in one hour. The duration of their effect is three to four hours. Insulin analogues provide an alternative to the more familiar, quick-acting clear insulins and can be used just before, or even immediately following a meal. Early evidence suggests that they may reduce the risk of severe, nocturnal hypoglycaemia in some people and improve overall glycaemic control. However, they are not suitable for everyone, and in some circumstances their rapid action can be a disadvantage and so they are always used cautiously. A well-known type of lispro insulin is called Humalog.
Quick-acting Soluble or Clear Insulin
This is the traditional type of rapid-acting insulin of which there are several different formulations. Their number include Hypurin Bovine Neutral, Hypurin Porcine Neutral, Pork Velosulin, Humulin S, Human Velosulin and Human Actrapid. These begin to work in about 30 minutes and reach a peak of activity in one to three hours. Their duration of action is about four to eight hours, and soluble insulins should ideally be injected half an hour before a meal is to be eaten. They are mainly used to regulate the meal-time glucose intake and are generally superimposed upon 'background' injections of longer-lasting types of insulin.
Intermediate-acting Cloudy Insulins
These take a longer time before they begin to work and have a more prolonged period of activity. There are two types.
- Isophane Insulins: These are among the most widely used types and preparations include Hypurin Bovine Isophane, Hypurin Porcine Isophane, Pork Insulatard, Humulin 1 and Human Insulatard. They begin to take effect after about two hours, with a peak of activity lasting between four and twelve hours. The maximum duration of their effect is between 22 and 24 hours, and isophanes are used to provide 'background' or basal insulin, usually in combination with quick-acting preparations.
- Lente Insulins: These have similar properties and examples include Hypurin Bovine Lente, Humulin Lente and Human Monotard. These begin to exert an effect after about two hours with a peak of activity lasting for between six and fourteen hours. Like isophanes, the total duration of effects is around 22 to 24 hours and they are usually used in combination with fast-acting preparations.
Long-acting Cloudy Insulins
These are the longest-lasting insulin preparations and examples include Hypurin Bovine Protamine Zinc, Hypurin Bovine Lente, Humulin Zn and Human Ultratard. These are more variable than other insulins with a peak of activity ranging between eight and twelve hours and total effects that may last up to 28 hours.
Insulin Mixtures
These are pre-mixed preparations, usually of quick (clear) and intermediate (cloudy) insulins. Most mixtures consist of a combination of clear and cloudy isophane insulins but, less commonly, lente insulin may be used as the longer-acting component.
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