Different preparations of insulin provide a range of options in terms of how quickly they take effect, their peak time of action and their overall duration of effect:
- Rapid-acting insulin analogs have an onset of action at between 5-15 minutes, a peak action at 30-90 minutes and an overall duration of effect of 3-5 hours
- Short-acting, regular insulin has an onset of action at between 30-60 minutes, a peak action at 2-3 hours and an overall duration of effect of 5-8 hours. The optimum time for injecting is 30 minutes before eating
- Intermediate-acting insulin has an onset of action at between 2-4 hours, a peak action at 4-12 hours and an overall duration of effect of 10-18 hours
- Long-acting insulin has an onset of action at between 2-10 hours, a peak action at 6-16 hours (except insulin glargine, which has no peak) and an overall duration of effect of 16-24 hours. These insulins maintain glucose levels fairly uniformly over a 24-hour period.
Side effects of insulin that are more common include:
- Hypoglycemia - low blood sugar levels that can result from the timing of the insulin injection. Hypoglycemia might be avoided by shifting a pre-dinner dose of intermediate-acting insulin to bedtime, or reducing a bedtime dose
- Weight gain - this may happen initially when insulin therapy is started, due to correction of protein and energy metabolism. Later weight gain may be caused by fluid retention or excessive eating due to hypoglycemia
- Lipohypertrophy - raised lumps in the skin caused by repeated injections at the same site; this is can be prevented by the rotation of injection sites
Other local effects - these are less common than lipohypertrophy and include infection, injection site abscess (both of which can be prevented with good injection practices), allergy and lipoatrophy (loss of fat tissue).