There are more than 20 types of insulin products available in four forms, according to the U.S. Food and Drug Administration (FDA). These forms are:

• Rapid-acting insulin. Known as Lispro (Humalog) or Novolog, it begins working within 15 minutes.
  
  
• Short-acting insulin. Known as “regular” insulin, it takes 30 to 45 minutes to work.
  
  
• Intermediate-acting insulin. Known as NPH or Lente (no longer available in the United States), it is mixed with a substance that causes the body to absorb the insulin more slowly. It begins to work within two to four hours.
  
  
• Long-acting (basal) insulin. Known as Lantus or Levemir, it acts in a way that is more or less stable for the whole day.

The type of insulin that is best for a patient depends on several factors, including:

• Onset of action. When the insulin starts lowering glucose (blood sugar) levels.
  
  
• Peak of action. When the insulin provides the maximum effect on glucose levels.
  
  
• Duration of action. How long the effect lasts.
  
  
• Health requirements of the patient, which include glucose levels, ability to perform glucose monitoring, exercise, diet and other medications.

In addition, there are a number of factors individual to each patient that may influence the type of insulin appropriate for treatment, including:

• Individual response to insulin. The period of time it takes for insulin to be absorbed and the length of time it remains active vary from patient to patient.
  
  
• Lifestyle choices. Diet, alcohol use and exercise patterns all affect how the body processes insulin.
  
  
• Level of comfort with multiple injections.
  
  
• Willingness to check glucose level frequently.
  
  
• Age.
  
  
• Glucose management goals.

Insulin derived from animal pancreases first became available to diabetic patients in the 1920s. Genetically engineered synthetic insulin, which is similar to natural human insulin, has in recent decades largely replaced insulin obtained from cows and pigs.

The same source of insulin (human or animal) made by the same manufacturer should be used, to ensure consistency in insulin delivery and effect.