The US Food and Drug Administration has added a boxed warning to ferric carboxymaltose, an intravenous iron treatment, because it can sometimes cause dangerously low phosphate levels in the blood.
The FDA announced the change on September 1, 2026.
A boxed warning is the most prominent safety warning used in US prescription medicine labeling.
The warning does not mean that the drug should no longer be used. Instead, it highlights a complication that doctors need to watch for, especially in people receiving repeated doses.
What is ferric carboxymaltose?
Ferric carboxymaltose is a form of iron given through a vein.
It is used when someone has iron deficiency and oral iron is unsuitable or has not worked well enough.
It can also be used in some people with chronic kidney disease and iron deficiency associated with heart failure.
Intravenous iron can replace iron more quickly than tablets and remains an important treatment when used appropriately.
What is phosphate, and why does it matter?
Phosphate is a mineral that the body needs for normal energy production, muscle function and strong bones.
If the phosphate level falls too low, a person may develop symptoms such as fatigue, muscle weakness or pain, confusion, seizures, or abnormal heart rhythms.
If low phosphate continues for a long time, it can also weaken bones and increase the risk of fractures.
Some symptoms, particularly tiredness and weakness, can resemble the symptoms of iron deficiency itself.
Why has the FDA strengthened the warning?
The risk of low phosphate with ferric carboxymaltose was already known.
The FDA says it had strengthened warnings several times in previous years.
However, further review in 2026 found that serious cases were still being reported.
Some patients required hospital treatment, prolonged phosphate replacement and repeated medical monitoring.
The FDA also found that phosphate testing was being performed in fewer than 20% of treatment episodes in the data it reviewed.
That combination of continuing serious cases and low monitoring led the FDA to add the boxed warning.
Who may need closer monitoring?
The FDA recommends checking phosphate levels before another course of ferric carboxymaltose in people who are at increased risk or who are receiving another course within three months.
Any existing low phosphate level should be corrected before treatment.
People who repeatedly develop severe or persistent low phosphate may need a different treatment plan.
Does this mean IV iron is unsafe?
No.
This warning relates specifically to the known risk associated with ferric carboxymaltose.
It does not mean that all intravenous iron treatments are unsafe.
And patients should not stop prescribed iron treatment on their own.
Iron deficiency can itself cause serious problems, and treatment can be highly beneficial.
The important change is that doctors are being urged to pay more attention to phosphate levels in people at risk.
What should patients know?
Someone who has recently received ferric carboxymaltose should seek medical advice if they develop new or worsening symptoms such as marked muscle weakness, persistent fatigue, bone pain, joint pain or confusion, especially after repeated iron infusions.
These symptoms can have many causes.
A blood test is needed to confirm whether phosphate is actually low.
Why this matters
Drug safety information often develops further after a medicine has been used in much larger numbers of people.
Clinical trials before approval cannot identify every uncommon side effect.
That is why regulators continue monitoring medicines after they enter routine use.
In this case, the risk itself was already known.
The new development is that the FDA now believes the warning needs to be made much more prominent.
Evidence so far
Evidence strength: Regulatory safety action
The FDA based its decision on several types of post-marketing safety information, including adverse-event reports, published research and large healthcare monitoring data.
These sources are especially useful for identifying safety problems after a medicine has been widely used.
