The first treatment offered to most patients with iron deficiency is an oral iron supplement. It's cheap, widely available, and unremarkable to prescribe. For some patients it works beautifully — and for those people, an intravenous infusion would be overkill.
But oral iron has a high failure rate that is rarely discussed in primary-care appointments. By the time many patients reach our clinic, they have been on iron pills for six, twelve, or twenty-four months with little to show for it except gastrointestinal side effects and a ferritin that has barely moved. The question is not "which is better." The question is "which one is right for this patient, today."
How oral iron is supposed to work
Oral iron supplements deliver iron through the digestive tract. The iron has to dissolve in the stomach, travel to the duodenum, cross the intestinal lining via specialised transporters, and then enter the bloodstream where it binds to transferrin and is delivered to the bone marrow.
This pathway has been built into human biology for good reason: it tightly regulates iron absorption to prevent overload. But it also imposes a ceiling. In most adults, only a small fraction of an oral iron dose is actually absorbed — typically less than 10 percent. The rest passes through the gut, where it can cause nausea, constipation, dark stools, abdominal cramping, and a metallic taste. For patients with inflammatory bowel disease, celiac disease, or prior gastric surgery, absorption can be lower still.
Where oral iron fails
In our experience, oral iron most commonly fails in five scenarios:
- Intolerance. The GI side effects are severe enough that the patient cannot take the dose reliably.
- Malabsorption. Celiac disease, inflammatory bowel disease, atrophic gastritis, post-bariatric anatomy, or chronic PPI use impair iron uptake.
- Ongoing blood loss. Heavy menstrual bleeding, GI bleeding, or frequent blood donation outpace what oral iron can replace.
- Severe deficiency. Ferritin in single digits with significant anemia. The deficit is simply too large to close with pills.
- Time-sensitive need. Pre-operative optimisation, late-pregnancy correction, or symptomatic patients who cannot wait the four to six months oral iron typically requires.
What IV iron does differently
Intravenous iron bypasses the gut entirely. A solution containing a stable iron-carbohydrate complex is infused directly into the bloodstream, where it is taken up by macrophages and gradually released into circulation over the following days and weeks. Because there is no absorption step, the dose limitation of oral iron disappears: a single 1000-mg infusion can deliver as much iron as a year of pills, and 100 percent of it reaches the body.
That has practical consequences. Patients who have been languishing on oral iron for months often see ferritin rise from single digits into the hundreds within weeks of a single infusion. Energy, mood, and concentration improvements commonly start within the first week and continue building over two to four weeks as red blood cell production catches up.
The three formulations we use
At MED1 we work with three modern IV iron products, all Health Canada approved:
- Monoferric® (ferric derisomaltose). A high-dose, single-session formulation. Most patients can receive their entire calculated iron deficit in one infusion of 15–30 minutes. Excellent tolerability profile.
- Ferinject® (ferric carboxymaltose). A widely used high-dose option, also typically administered in one or two sessions. Strong safety record.
- Venofer® (iron sucrose). An older formulation requiring multiple smaller doses. We use it selectively — for example, in patients with prior reactions to newer agents.
Selection is made by the supervising internist based on the patient's iron deficit, prior reactions, kidney function, and pregnancy status. For most patients, the answer is a single Monoferric or Ferinject session.
Safety, side effects, and what to expect
Modern IV iron formulations are very well tolerated. The most common side effects are mild — transient flushing, headache, slight nausea, or a feeling of warmth during the infusion. These usually resolve within minutes. Serious reactions are rare and, when they occur, are almost always managed without consequence in a properly monitored clinical setting. This is why every infusion at MED1 is delivered with a Registered Nurse administering, a physician on site, and full pre-, intra-, and post-infusion monitoring.
A small number of patients experience a transient drop in serum phosphate after Ferinject in particular. We monitor for this in patients with risk factors and adjust the choice of formulation when appropriate.
When oral iron is still the right call
None of this means oral iron is obsolete. For many patients — particularly those with mild deficiency, intact gut function, no time pressure, and good tolerance of pills — oral iron is the first-line answer and a single course is enough. A well-conducted course of oral iron can absolutely replace IV iron in the right patient, particularly when paired with absorption-optimised dosing (alternate-day dosing, taken on an empty stomach with vitamin C, away from coffee, tea, calcium, and PPIs).
The decision between oral and IV iron is not a moral one. It is a match-the-treatment-to-the-patient one. Both belong in the toolbox.
How we decide, in practice
At a typical Internal Medicine consultation, the decision rests on three questions: how big is the deficit, what's the cause, and how fast does it need to be corrected? A 30-year-old with mild deficiency from a vegetarian diet may do beautifully with three months of oral iron and dietary adjustment. A 45-year-old with heavy periods, ferritin of 4, profound fatigue, and an upcoming surgery is a much better candidate for a single IV infusion. Most of our patients fall somewhere in between — and the consultation is where that judgement happens.
If you have been on oral iron without progress, or if your situation suggests an infusion may be the right call, our Internal Medicine Clinic handles diagnosis and decision, and the Infusion Suite handles treatment — under the same physician, in the same building, typically within the same week.