If you have Type 1 diabetes, your doctor prescribes insulin to replace the insulin your body is unable to produce.

Insulin is sometimes prescribed for persons with Type 2 diabetes when diabetes pills and lifestyle changes failed to control blood glucose levels. It is also prescribed in gestational diabetes when diet alone is unable to control blood glucose1.

Understanding how insulin works can help you better manage your condition and prevent serious problems.

How Insulin Works

how insulin works

Insulin controls blood glucose in two ways:

  • Insulin moves glucose from the blood into the cells
    After a meal, your body breaks down the carbohydrates (starches and sugars) in the food into a type of sugar called glucose. Glucose enters the blood to be used by body cells for energy. Insulin’s job is to help glucose enter the cells.
  • Insulin stores excess glucose
    When there is too much glucose in the blood, insulin helps to store the extra glucose in the liver for future use. When the blood glucose level is low, such as in between meals, the liver releases extra glucose back into the blood to help keep blood glucose levels within the normal range.

In Type 1 diabetes, the body produces little or no insulin. For Type 2 diabetes, the body does not produce enough insulin or the insulin is not working properly. In both cases, the absence or ineffectiveness of insulin causes blood glucose levels to spike. Prolonged high blood glucose can lead to serious problems like blindness, nerve damage and kidney failure.

Types of Insulin

types of insulin

There are many types of insulin. Some work slowly and some quickly.

  • Slow-acting insulin releases a steady supply of insulin that controls blood glucose levels over a longer time.
  • Fast-acting insulin, on the other hand, acts quickly to control blood glucose levels. You need fast-acting insulin when you are sick, under stress (undergo operation) or have a high blood sugar problem.

This table explains the different types of insulin. Depending on your condition and lifestyle, your doctor will decide which types of insulin you need and how much you should take each time. Remember to follow your doctor’s instructions closely so that insulin treatment works for you.

Type of Insulin

Common Insulin Names

When to Take it

How Soon It Starts to Work

How Long Does It Last

Rapid-acting insulin




Right before a meal

12-30 minutes

2-5 hours

Short-acting insulin

Humulin R


30 minutes before a meal

30-60 minutes

6-8 hours

Intermediate-acting insulin


Humulin N

Before breakfast and/or at bedtime

1-4 hours

12-20 hours

Long-acting insulin



Daily at the same time (e.g. bedtime)

1-4 hours

18-24 hours

Premixed mixture of rapid-acting and medium-acting insulin

Humalog Mix 25/75

NovoMix 30/70

Right before breakfast and/or right before the evening meal

10-20 minutes

10-16 hours

Premixed mixture of short-acting and medium-acting insulin

Mixtard 30

Humulin 30/70

Mixtard 50

Humulin 50/50

30 minutes before breakfast and/or before the evening meal

30-60 minutes

10-16 hours

Source: Ministry of Health Singapore (2014). Diabetes MOH Clinical Practice Guidelines 1/2014.

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