Iron IV
Not all iron infusions are the same.
Two drips can carry the same iron and leave your bones in very different places. What nobody explains before you sit down in that chair.
If you are a woman with iron-deficiency anaemia, there is a good chance the cause is your period, and there is a good chance that nobody offered you a choice about which iron infusion you received.
For years, I've been trying to get an iron IV. Literally, for over twenty years. I kid you not. The Netherlands likes to think of itself as a koploper when it comes to healthcare. It isn't, not really, not here. Iron infusions haven't been around that long in this country, and for a long time you could only get one when it was deemed absolutely necessary. Behind that word "necessary" stands a long line of women with iron problems who never cleared the bar — because the lab values that define iron deficiency are set unreasonably low for women compared to men. Without, as far as I can tell, any real justification behind that difference. A difference that is still upheld, still unquestioned, even though the critique has been raised more than once. But. Finally, I'm getting it. And I'm getting it only after agreeing to their terms — which included stopping my period. If you know me, you know I was never going to agree to that quietly. So I asked for a good reason. What I got, and I promise I'm not exaggerating, was my doctor telling me she didn't think hormones were all that important in a woman's life — more of an inconvenience than anything. And then, in the same breath, that the infusion I was finally being given would come at a cost to my bones, and that I'd need it regularly. So you do the math. That last part — the bones — is what this whole post is about. Because of course I went and checked what she told me. For my next appointment. And for yours.
So, iron infusions get talked about as if they're one thing, a bag of iron, hung on a pole, dripped into your arm, done. But they are not one thing. There are several different formulations, and the differences between them are not trivia. One of them, the one most widely used, carries a specific and well-documented risk to your bones that the others largely don't. And the group most likely to receive it is women, because the most common reason a woman needs iron fast is heavy menstrual bleeding.
This is the post I wish existed before anyone I know sat in that chair. Not to frighten you off infusions, which are genuinely important and sometimes necessary, but so that you can walk in knowing what to ask.
The two names worth knowing
Modern high-dose iron infusions mostly come down to two molecules. They deliver comparable iron. They are not comparable in what else they do.
The first is ferric carboxymaltose, sold in Europe as Ferinject and in the US as Injectafer. It's popular for good reasons: you can give a large dose quickly, often in one or two visits, which is convenient for everyone involved.1 The second is ferric derisomaltose, formerly called iron isomaltoside, sold as Monofer or Monoferric. It too delivers a high dose in few visits.
On the surface they look interchangeable. The difference sits in what happens to a hormone you've probably never heard of, and through it, to the mineral your bones are built from.
The pattern you're seeing is real and it's backed by head-to-head trials, not marketing. When ferric carboxymaltose and ferric derisomaltose were compared directly in randomised studies, the carboxymaltose caused significantly more of a problem called hypophosphataemia, a drop in blood phosphate, and caused it more often and for longer.2 That word is the hinge of this whole story, so let me take it apart.
How an iron drip reaches your bones
Phosphate is not a footnote mineral. Alongside calcium, it's one of the two building blocks your body uses to mineralise bone, to make it hard. Run low on phosphate for long enough and bone can't mineralise properly. So the chain that matters runs: iron infusion, to a hormone, to phosphate, to bone. Here's each link.
The name for the end of that road is osteomalacia, and it's worth being precise, because this is where a lot of online writing gets sloppy. Osteomalacia is not the same as osteoporosis. Osteoporosis is bone that has thinned, lost quantity. Osteomalacia is bone that is soft because the new bone your body lays down can't harden properly for lack of phosphate. Different mechanism, different problem. What repeated ferric carboxymaltose has been linked to, in case reports and now animal work, is the softening kind: bone pain, and in severe cases the insufficiency fractures that follow.3 Calling it decalcification isn't quite right, and precision here is part of being trustworthy. It's a mineralisation failure, and phosphate is the mineral in question.
Why this lands on women, specifically
Now the part that makes this a women's health story rather than a pharmacology aside.
The reported incidence of ferric-carboxymaltose-induced hypophosphataemia isn't rare. Depending on the population it ranges widely, but in women with iron-deficiency anaemia caused by heavy menstrual bleeding, one line of research put it as high as around seventy percent.4 Read that again. The single most common reason a woman ends up needing an iron infusion, heavy periods, is also the setting where this particular side effect shows up most.
And who gets iron infusions? Overwhelmingly women. Iron deficiency is far more common in menstruating and pregnant bodies. So the preparation with the highest phosphate-and-bone risk is being given, disproportionately, to exactly the population most susceptible to that risk, often more than once, sometimes cycle after cycle, year after year. The repetition matters: the bone effects in the research grow with repeated dosing, which is why the honest guidance is to use as few infusions as the clinical situation allows.5
A known risk, a Dutch safety notice, and still no conversation at the chair
This is not fringe or emerging. The Dutch pharmacovigilance centre Lareb issued a signal on intravenous iron and osteomalacia years ago. The product information for ferric carboxymaltose itself names the risk of hypophosphataemia, hypophosphataemic osteomalacia, and fractures, and recommends monitoring in people who receive multiple doses long-term.6 The knowledge exists, in the regulatory documents, in the trials, in the leaflet inside the box.
And yet the woman in the infusion chair is almost never told there's more than one option, or asked whether her phosphate will be checked afterward, or warned that if she comes back for repeat infusions the bone question grows. She is handed the convenient, widely-stocked default and sent home. The gap here isn't scientific. It's a conversation that doesn't happen, in a system that, as I've written before, is built to deliver treatment efficiently rather than to weigh the slower, downstream costs that fall on the patient later.
I want to be fair and clear: ferric carboxymaltose is a legitimate, approved, often necessary medicine, and for many people a single infusion is completely fine, with any phosphate dip resolving on its own. This is not a reason to refuse iron you need. Untreated iron-deficiency anaemia is itself serious. It is a reason to be an informed participant rather than a passive recipient.
What to actually do with this
- Ask which preparation you're getting, by name. Before an infusion, it's entirely reasonable to ask: is this ferric carboxymaltose (Ferinject/Injectafer) or ferric derisomaltose (Monofer)? You're not second-guessing your clinician by knowing the name of your own medicine. You're being a participant in the decision.
- Raise phosphate, especially if you'll have more than one. If you're getting repeat infusions, or you already have risk factors like vitamin D deficiency or a gut condition affecting absorption, ask whether your blood phosphate can be checked in the weeks afterward. A simple blood test catches the thing that, left unseen over time, is what leads to bone trouble.
- Know the symptoms that deserve follow-up. New, unexplained bone pain, muscle weakness, or fatigue that lands differently than your usual anaemia tiredness, in the weeks and months after infusions, is worth flagging to your doctor specifically in the context of the iron you received. Most dips are mild and silent; the ones that aren't deserve attention.
- Treat the cause, not only the deficiency. If heavy menstrual bleeding is why you keep running low on iron, the infusion treats the symptom while the tap stays open. It's worth a real conversation about why the bleeding is heavy, because addressing that can reduce how often you need infusions at all, which is its own kind of bone protection.
- Don't let this scare you away from iron you need. The point of knowing all this is better questions, not fear. Iron-deficiency anaemia is genuinely worth treating, and an infusion can be the right call. Informed and treated beats frightened and depleted, every time.
This is education, not medical advice, and certainly not a recommendation for or against any specific medicine. Which iron preparation is right for you depends on your full clinical picture and is a decision for you and your doctor. Nothing here should replace that conversation.
The thing I keep coming back to is how small the ask is. Nobody needs a medical degree to say the name of their own medicine out loud and ask what it does. The information gap here isn't vast or technical. It's the width of a single question that, for reasons of habit and hurry, tends not to get asked.
Two bags of iron can look identical hanging on the same pole. What they leave behind, over years, over repeat visits, in a body that menstruates and comes back for more, is not identical. You are allowed to know that. You are allowed to ask.
Informed is not the same as afraid. It's just the difference between something being done to you and something being decided with you. In your own care, that difference is most of what I'd wish for you.
So much of taking your own health seriously is this: learning the questions nobody hands you, in a system that assumes you won't ask. That posture, informed, participating, unwilling to be a passive recipient, runs through all of The Art of Female Health, if this is the muscle you want to build.
- Wolf, M., et al. (2020). Effects of iron isomaltoside vs ferric carboxymaltose on hypophosphatemia in iron-deficiency anemia: two randomized clinical trials. JAMA, 323(5), 432–443. doi:10.1001/jama.2019.22450
- Schaefer, B., et al. (2022). Choice of high-dose intravenous iron preparation determines hypophosphatemia risk. Head-to-head evidence summarised in JCEM. Journal of Clinical Endocrinology & Metabolism, 107(4), 1009–1019. academic.oup.com/jcem
- Zoller, H., et al. (2023). Hypophosphataemia following ferric derisomaltose and ferric carboxymaltose (PHOSPHARE-IBD): a randomised clinical trial — FCM linked to osteomalacia, fractures and muscle weakness; FDI not. PMC10086283
- Distinct roles of ferric carboxymaltose and ferric derisomaltose on phosphate homeostasis (2025) — reported FCM-hypophosphataemia incidence up to ~70% in women with IDA from heavy menstrual bleeding. PMC12924155
- Iron overload induced by ferric derisomaltose and ferric carboxymaltose both increase FGF-23 and lead to osteomalacia and bone loss in mice (2026) — greater bone volume loss with FCM; effects grow with repeated dosing. BioMetals. doi:10.1007/s10534-026-00794-x
- Lareb (Netherlands Pharmacovigilance Centre, 2019). Signal: Intravenous iron and osteomalacia — Ferinject and Monofer; product information notes hypophosphataemic osteomalacia and fracture risk with long-term repeated dosing. lareb.nl