Iron infusion side effects: what's common, what's rare, and what to watch for

Iron infusions have an excellent safety record when they're given to the right person, for the right reason, by trained clinicians. But 'generally safe' isn't the same as 'nothing to know'. Informed consent means understanding what's common, what's rare, and what to do if something doesn't feel right - so here is the honest rundown, based on current New Zealand guidance for ferric carboxymaltose (Ferinject).

Common and usually mild

• Headache, nausea or dizziness - reported by roughly 3-7% of people for nausea, and around 1% for headache and dizziness. These usually settle quickly.
• Mild irritation, bruising or discomfort at the drip site.
• A temporary metallic taste during the infusion.

Delayed flu-like symptoms

Some people feel mildly unwell one to two days after their infusion - a low-grade fever, aching joints or muscles. This can be surprising if nobody warned you, but it's a recognised effect, it typically passes within a day or two, and simple rest and fluids are usually all that's needed. If a fever is high or you feel significantly unwell, seek medical advice rather than assuming it's the infusion.

Low phosphate (hypophosphataemia)

Ferinject can temporarily lower blood phosphate levels - this was the subject of a December 2024 Medsafe reminder to prescribers. It's usually mild and symptom-free, but can occasionally be more significant, particularly with repeated infusions. Ongoing tiredness, bone pain or muscle weakness in the weeks after an infusion are the symptoms to mention to your GP.

Skin staining

If iron leaks outside the vein during the infusion, it can leave a long-lasting brown stain on the skin. This is uncommon and largely preventable with good cannulation technique and monitoring - which is why your paramedic checks the drip site throughout, and why you should say something straight away if you feel pain, swelling or wetness at the site.

Rare: allergic reactions

Serious hypersensitivity (allergic) reactions are rare - in the order of 1 in 1,000 - but they are the reason iron infusions should never be a casual, unsupervised affair. Your paramedic carries the equipment and training to recognise and manage a reaction, and you'll be observed for 30 minutes after your infusion before we pack up.

When to seek advice afterwards

• Difficulty breathing, facial swelling or a widespread rash - call 111 immediately.
• A high fever or feeling significantly unwell beyond mild flu-like symptoms.
• Increasing pain, swelling or discolouration at the drip site.
• Ongoing unexplained tiredness, bone pain or muscle weakness in the following weeks - see your GP and mention the infusion.