U500 insulin: a potential safety issue   (September 9, 2008)

The FDA just released its first quarterly report of drugs with "potential safety issues." A discussion of the concept is posted at FDA to Post Quarterly Report of Potential Safety Issues, and the first report is for the first three months of 2008.

As the FDA points out, "The information is being provided under provisions of the Food and Drug Administration Amendments Act, signed into law Sept. 27, 2007. The law requires that FDA inform the public each quarter of new safety information or potential signals of serious risk, based on the agency's review of adverse event reports... The appearance of a drug on this list does not mean that FDA has concluded that the drug has the listed risk, or that FDA has identified a causal relationship between the drug and the listed risk. It is on the list only because FDA has identified a potential safety issue."

I glanced through the list of suspects, and was a bit surprised to find an insulin product on the list: Insulin U-500 (Humulin R). The "Potential Signal of Serious Risk/New Safety Information" is described as "dosing confusion."

Humulin R is the brand name for Eli Lilly's recombinant human regular insulin. It's been on the market in the US since 1982 (See Celebrating a Milestone: FDA's Approval of First Genetically-Engineered Product). The usual version of Humulin R is U-100; that is, 100 units of insulin activity per 1.0 cc of fluid.

But there's also a rarely-used version called U-500 which contains 500 units of insulin activity per 1.0 cc. It's used in extreme insulin resistance scenarios. The U-500 version of regular insulin lasts a lot longer (up to 24 hours, per the official patient information). Accidentally getting the U-500 version is a definite danger, not only of knocking the blood sugar level down much lower than expected, but of having prolonged hypoglycemia, as this version hangs around a lot longer than U-100 would be expected to last. Definitely a safety issue!

The Institute for Safe Medication Practices has described an increase in reports about mixups between insulin U-100 and insulin U-500. They point out that mistakes have occurred when prescribers accidentally selected U-500 regular insulin from computer screens instead of U-100. ISMP described possible reasons for these kinds of errors. "For example, sometimes the two dosage forms appear one line apart on the screen, which makes it easy to select the wrong one. Also, depending on the screen size, the prescriber may see only the first few words of the product listing, so the drug concentration may not be visible. And finally, since the use of U-500 insulin is not common at present, prescribers may just assume that the only regular insulin that's available is U-100."

What to do? Well, first of all, Humulin R (U-500) is the only human insulin by Eli Lilly and Company that has a concentration of 500 units/mL. Novo-Nordisk has also made a highly-concentrated regular insulin, with 400 units/mL, but it doesn't seem to be available in the US presently. So unless you're using regular insulin, you won't have this problem. But if you are, be aware that there are various strengths available: U-100 being by far the most commonly-used, and U-500, which is only rarely prescribed. I contacted Lilly, and they say you can visually tell them apart both by the wording "U-500" and distinctive diagonal stripes on both the carton and the bottle, that aren't on the U-100 version.

As the label points out, always check the carton and the bottle label for the name and letter designation of the insulin you receive from your pharmacy to make sure it is the same as that your doctor has prescribed.

Sounds like a good idea no matter what medication you're taking!