Insulin comes in several different strengths and actions. Rapid-acting insulin such as Humalog, Novolog, or Apidra is injected before a meal and starts working in fewer than 15 minutes. Its peak of action (when it is working the hardest) is between 30 to 90 minutes after injection, about the time when blood sugar levels would be at their height after a meal.

Regular insulin starts working 30 to 60 minutes after injection and peaks later than rapid-acting insulin. NPH insulins are longer lasting with a slower onset and peak action, and they are often used in combination with regular insulins.

Insulin mixes can also be used to “cover” a meal; the most common premix is 70/30 (NPH and regular insulin). Newer long-acting insulins such as insulin glargine (Lantus) and detemir (Levemir) are designed to provide up to 24-hour “peakless” coverage.

It's important to note that onset, peak, and duration vary from person to person, and can be influenced by other health conditions you may have, as well as weight. You and your doctor will choose an appropriate insulin or mix of insulins based on your particular needs and blood glucose patterns.

The insulin mixes (i.e., 70/30, 75/25, 50/50) are convenient, commonly prescribed insulin combinations. They eliminate any operator error with trying to draw up two different insulin types, and are a boon to those people who may have vision or dexterity problems or find mixing insulins difficult for other reasons.

Insulin Dosage

Your insulin dose will depend on the type and action of your insulin. Your doctor will tell you exactly what kind and how much insulin to inject, and when you should be taking it. However, the amount of regular or rapid-acting insulin taken before meals will have to be calculated “on the fly” based on how many carbohydrates you are going to eat. You'll have to do a little bit of math to determine your dose.

Premeal insulin dosing is based on something called the insulin-to-carbohydrate ratio. Everyone has their own unique ratio, but the rule of thumb is that each unit of regular insulin covers about 15 grams of carbs (the insulin-to-carbohydrate ratio). For every 50 mg/dl blood sugar is above the target range, an additional unit of insulin should be added (the blood sugar-to-insulin ratio). So if you were planning on a dinner of 90 carbs and your blood sugar was 100 mg/dl over target, 8 units of insulin would be necessary (6 to cover the carbs and 2 for the blood sugar).

Remember, this is a general guideline only; the insulin-to-carb ratio is different for everyone, as is the insulin-to-blood sugar ratio, and you should work with your diabetes health care provider to track your glucose levels and determine what ratios are right for you.