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If you’re about to get an iron injection to treat your iron deficiency anemia, there’s one question you should be sure to ask your doctor: “What kind of iron are you using?”

You would think that all forms would be the same, but recent studies have suggested that they’re not. In fact, one of them could greatly increase your risk of a serious, even potentially deadly, condition over others.

What Kind of Iron Injection Will I Receive?

Injectable forms or iron are often used to help quickly restore iron levels in patients with iron deficiency anemia—particularly in those patients who can’t tolerate oral iron supplements, who did not have a good response to them, or who have non-dialysis chronic kidney disease. They may also be used in patients who have extremely low levels of iron or who have lost a lot of blood.

Iron injections don’t have to go through the digestive system, so they can help replenish iron levels more quickly than iron pills. There are different types of iron solutions that can be injected either into the circulatory system (intravenously) or into the muscle to increase iron levels. Below are the most common ones:

  1. Iron Sucrose: This form is given intravenously, and is often used in patients with kidney failure because it’s less likely to trigger an allergic reaction. It is given only in smaller doses, usually over a period of days or weeks. One brand that uses this form is called Venofer.
  2. Iron Dextran: This form can be injected into muscle or administered intravenously. Some patients are allergic to it, though, and may not be able to tolerate it. An allergic reaction can be serious in rare cases, causing an anaphylactic reaction. Usually a test dose is done first. Brand names include INFed and Dexferrum.
  3. Sodium Ferric Gluconate: This form is given intravenously, and is used specifically in patients who are undergoing hemodialysis in conjunction with erythropoietin therapy for kidney failure. Most patients receive several smaller doses over a period of days or weeks. The brand name is Ferrlecit.
  4. Ferumoxytol: This form is also given intravenously, and is administered quickly in higher doses over a shorter period of time. Usually two doses are given to patients, with the second dose in three to eight days. The brand name is Feraheme.
  5. Iron Isomaltoside: This form is delivered intravenously, and consists of iron and chemically modified isomalto-oligosaccharides that help reduce the risk of allergic reaction and provide for a slow release of the iron inside the body. The brand name is Monofer.
  6. Ferric carboxymaltose (FCM): This form is given intravenously, and is administered in two higher doses separated by at least seven days. It contains iron stabilized by a carbohydrate shell, which reduces the risk of an allergic reaction. It also helps create a timed-release effect, so that the iron is released slowly in the body. The brand name is Injectafer.

All of these forms replenish iron stores in the body, but recent studies have found that they’re not all the same when it comes to side effects. Even between types that are quite similar—such as the extended-release Monofer and Injectafer—the side effects are quite different.

Will My Iron Injection Increase Risk of Hypophosphatemia?

Some studies have shown that Injectafer, more than other iron injections, can increase the risk of hypophosphatemia (HPP) and severe hypophosphatemia. These are conditions in which the body is deficient in phosphate, an important nutrient. At mild levels of deficiency, the symptoms may not be serious, but at moderate or severe levels, they can be life-threatening.

In 2016 researchers compared the iron in Injectafer (FCM) with the iron in Monofer (iron isomaltoside or IIM), and found that Injectafer caused more cases of HPP. In fact, the risk was 45.5 percent with those taking Injectafer, compared to only four percent with those taking Monofer.

Researchers concluded, “Treatment with FCM is associated with a high risk of developing severe and prolonged hypophosphatemia and should therefore be monitored. Hypophosphatemia risk appears to be substantially lower with IIM.”

Other studies have shown similar risks, which means patients need to ask their doctors three questions before receiving iron injections:

  • Which form of iron are you using?
  • Are you aware of the studies suggesting that some forms increase the risk of HPP?
  • Are you going to be monitoring my phosphate levels?

So far, the Injectafer manufacturers have not provided adequate warnings about the risks of HPP. That means it’s up to patients to advocate for their own health and safety.

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