Carolyn Robertson

Understanding Your Insulin Prescription

If you want to make sense of your sugars, it helps to know how insulin works.


Insulin is a vital hormone - without it, life cannot be sustained. It is needed continuously 24 hours per day - while awake and while asleep; during a meal; and during periods without food (fasting, overnight, etc). When it is deficient, it must be replaced. Unfortunately, the insulin that is available for patient use is not the same as the insulin that is produced by the non-diabetic individual. It must be injected — usually into the subcutaneous tissue; it is slow; and it does not respond to the body’s regulating system. As a consequence, the user must decide when, how often and how much to take. If you have Type 1 (Insulin Dependent Diabetes) Diabetes, you have lost the capacity to produce insulin. To survive, you need to replace the missing insulin by taking multiple injections of insulin every day or by using an insulin pump. In other words, you need an insulin prescription that gives you a steady level of insulin throughout the day and the night. You would also need the insulin prescription to provide larger quantities of insulin at meals.

If you have Type 2 Diabetes (Non-Insulin Dependent Diabetes), your insulin needs are very different. People with Type 2 diabetes often have the capacity to produce insulin. But, in many cases, the insulin levels eventually become either insufficient or ineffective. If this has occurred, you also require insulin therapy. Your insulin prescription is designed to supplement the missing or ineffective natural supply. It is given at those times when your insulin is unable to perform adequately. In other words, your insulin prescription is very individualized. You may only need insulin over night or you may only need some insulin at meal times. The exact insulin prescription is selected according to the specific needs of the individual. It is dependent on the blood sugar levels and the amount of residual effective insulin.


Knowing the activity curves of the different insulins is necessary to create an insulin schedule that truly meets the needs of the insulin user: the one that fits the individual’s unique insulin requirement, lifestyle and personal preferences. There are currently four categories of insulin activity available: Rapid acting insulin; Short acting insulin; Intermediate acting insulin; and Long acting insulin.

Rapid acting insulin or Humalog is the fastest acting insulin available. It begins to have an impact 15 minutes after injection; its peak effect usually occurs within 1to 2 hours; and it has duration of slightly more than 3 hours. It is usually taken immediately before a meal so its greatest impact occurs after that meal. Short acting insulin or Regular also has an early effect on the blood sugar. It, however, takes about 30 to 45 minutes before it begins to impact the blood sugar level; its peak effect takes 2 to 3 hours; and it can lasts for more than 4 to 8 hours. As a result, when Regular is taken at a meal, it must be taken at least 30 minutes before the start of the meal. To prevent an excessive glucose lowering, it usually requires that a snack be eaten 2 to 3 hours after the injection. Intermediate acting insulins and long acting insulins have a more delayed effect on the blood sugar levels.

Lente and NPH, both intermediate acting insulins, often take 1to 3 hours to have an effect on the blood sugar level; they have a delayed peak that varies from 4 to 10 hours; and a duration of 10 to 18 hours. When they are taken in the morning, they will have their greatest effect in the afternoon and often provide the individual with sufficient insulin to accommodate a meal 3 to 6 hours after the injection. The earlier peaking NPH usually requires an earlier lunch and the Lente, which peaks later, tolerates a later lunch. The prolonged action of both insulins often demand that an afternoon snack is eaten to prevent a low blood sugar reaction before supper. These same insulins taken in the evening will provide the individual with an overnight supply of insulin. However, the time they are taken will impact whether their greatest effect occurs in the middle of the night or just before waking. A sizeable bedtime snack is generally required when the insulin is taken in the early evening to minimize the risk of a low blood sugar overnight.

Ultralente, long acting insulin, has a subtle but more prolonged influence. It, too, can be taken in the AM or the PM but its impact lasts for 24 hours or longer. This insulin provides the low levels of insulin that the body requires in the absence of food. Lantus is a long lasting insulin analogue that should be available in September. It has been specially designed to provide insulin for a 24-hour period. Unlike Ultralente is does not have a significant peak. As a result, it is not likely to cause hypoglycemia. The dosing guidelines from the manufacturer suggest that this will be a once a day insulin taken at bedtime. However, it is likely that the actual usage may vary as physicians and nurse practitioners get experience with the insulin.

Name Type Starts (hrs) Best (hrs) Lasts (hrs)
HUMALOG RAPID ACTING 0.1- 0.5 1 — 2 3 - 4
REGULAR SHORT ACTING 0.5 — 1 2 — 4 6 - 8
LENTE MEDIUM ACTING 2 - 3 6 - 10 14 - 18
NPH (human) MEDIUM ACTING 1 — 2 4 — 8 10 -14
Percentages Insulin Mixtures
70/30 MIXTURE of 70%NPH & 30% REG
75/25 MIXTURE of 75% NPL* & 25% HUMALOG

*NPL is an intermediate insulin that is similar to NPH

50/50 MIXTURE of 50% NPH & 50% REG
  • Actual onset, peak and duration will vary from user to user.
  • Duration of insulin is also affected by the dose — the larger the dose, the longer the effect

Now that I have covered the types of insulin, I will go over insulin prescriptions in my June installment. Please remember that regardless of the prescription, no insulin regime can meet every need of every patient all of the time. In addition, individual factors such as site of injection, size of the dose, time of the day, amount and type of activity and even stress can alter an individual intensity of response. Self-monitoring of the blood sugar levels, self study and collaboration with a medical team that is willing to teach you about diabetes and help you learn the tricks are necessary to make winning combinations.

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