Insulin Overview for Pharmacists

When a patient begins using insulin to manage diabetes, the initial dose is just a starting point.

At first, insulin doses often must be adjusted based on titrating basal dosing and meal times. This is a great jumping off point, but over time, insulin requirements are affected by things like weight gain or loss, changes in eating habits, and the addition of other medications. The need for insulin often increases, and the dose must be readjusted to meet the new requirements.

Insulin is injected subcutaneously, meaning not very deep below skin level. Common injection sites include the stomach, buttocks, thighs, and upper arms. By rotating the site of injection, patients can avoid lipohypertrophy, a slight increase in growth or size of fat cells under the skin. When lipohypertrophy occurs, a soft, pillowy growth may form at the repeated-use injection site. Therefore, for reliable absorption rates and cosmetic purposes, it’s important to rotate insulin injection sites.

Insulin types vary depending on how quickly they work, when they peak, and how long they last.

As sliding scale insulin therapy has fallen out of favor, many experts recommend using basal insulin with mealtime insulin added when needed. Basal insulin is intended to keep blood glucose levels under control during periods of fasting or sleep. The 2 main types of basal insulin are intermediate-acting insulin (NPH, Humulin N, and Novolin N) and long-acting insulin. There are 2 kinds of long-acting insulins: detemir (Levemir) and glargine (Lantus).

Intermediate-acting insulin:

- Taken once daily

- Peaks 4 to 8 hours after injection

- Effects last up to 18 hours

- If injected at bedtime, will likely peak at predawn hours, when needed most

Long-acting insulin:

- Evenly lowers glucose levels

- Effects last up to 24 hours

To mimic the way a nondiabetic patient’s body would release insulin, bolus insulin (short-acting or rapid-acting insulin) must be administered to prevent the increase in blood glucose after eating meals.

Rapid-acting insulin (glulisine, lispro, and aspart):

- Taken at mealtime

- Begins to work in 15 minutes

- Peaks in about 1 hour

- Effects last up to 4 hours

Regular or short-acting insulin (Humulin R; Novolin R; and, for insulin pump, velosulin):

- Taken at mealtime

- Begins to work in 30 minutes

- Peaks in 2 to 3 hours

- Effects last up to 6 hours

Premixed insulins combine specific amounts of intermediate-acting and short-acting insulin in 1 bottle or insulin pen. These products, which include Humulin 70/30, Novolin 70/30, Novolog 70/30, Humulin 50/50, and Humalog Mix 75/25, are generally taken 2 or 3 times a day before mealtime.

Insulin is available in several strengths, although most insulin sold in the United States is U100, meaning there are 100 units of insulin in every mL of liquid. A 10-mL bottle of U100 insulin contains 1000 units of insulin. There are 3 sizes of U100 insulin syringes: 1/3 cc, ½ cc, and 1 cc. A 1/3 cc syringe holds 30 units of insulin, ½ cc holds 50 units, and 1 cc holds 100 units. The type of syringe used depends on how much insulin is required.

Several new insulins with concentrations >U100 have recently come to the market. These products were developed for those requiring large doses of insulin to reduce the volume injected and the number of injections. Toujeo (insulin glargine [rDNA origin]), a once-daily, long-acting basal insulin, contains 300 units/mL (U300). For patients with severe insulin resistance in need of >200 units/day, the super-concentrated U500 insulin is available.

A variety of insulin delivery systems exist, including syringes, insulin pens (disposable and reusable), insulin pumps, needle-free jet injectors using very high pressure to push a fine spray of insulin through the skin, and inhaled insulin.