Iron preparations generally contain one of three iron salts: iron sulphate, iron gluconate, and iron fumurate. It is important to realize, however, that a tablet of the sulfate salt contains twice the amount of iron as a tablet of the other two salts, although the differing molecular weights of the compounds obscure this fact (). Therefore, twice as many ferrous gluconate or ferrous fumurate tablets are required to provide the amount of elemental iron in ferrous sulfate tablets.
Common oral iron preparations.
The choice of delivery formulation is another source of confusion. Oral iron may be given as tablets or elixirs. Among the tablet preparations, there are non-enteric coated pills and enteric-coated and prolonged-release formulations. Non-enteric coated iron tablets are most commonly used as initial treatment due to their lower cost. Delayed release and enteric-coated iron preparations have been advocated since they are better tolerated than the non-enteric coated tablets. However, they are less effective since they may contain less iron and their iron may not be released in the duodenum, where iron is absorbed. In fact, patients who have been treated unsuccessfully with enteric-coated and prolonged-release iron preparations may respond well to the administration of nonenteric-coated ferrous salts.12
There are multiple variables that may enhance or inhibit the absorption of medicinal iron (). Differences in absorption are most likely due to the requirement of acidity in the duodenum and upper jejunum for iron solubility. For iron released beyond these sites, the alkaline environment reduces absorption.13
Ideally, patients should not take iron supplements within 1-2 hours of antacids. The inhibition of iron absorption by other medications that reduce stomach acid like H2 blockers may be even more prolonged. Absorption is also delayed with tetracyclines, milk, and phosphate-containing, carbonated beverages such as soft drinks. Even the calcium, phosphorus and magnesium salts contained in iron-containing multivitamin pills impair absorption of elemental iron.14
For this reason, multivitamin preparations should never be recommended as a sole therapy for iron deficient anemia. Iron tablets are recommended between meals or at bedtime to avoid the alkalinizing effect of food and to take advantage of the peak gastric acid production late at night.
A common generic approach for iron deficiency in adults consists of a daily dose of 150-200 mg of elemental iron. This approach entails prescribing one ferrous sulfate tablet 3 times daily since each tablet contains approximately 60 mg of elemental iron. Assuming that 10% of the iron is absorbed, the hemoglobin concentration may fully correct after 4 weeks in patients with moderate, uncomplicated iron deficiency (about 500–800 mg of iron, enough for 500 to 800 mL of packed red blood cells, or enough to raise the whole blood hemoglobin 2–3 g/dL).15
To further replenish iron stores, some recommend continuation of this regimen for several additional months.16
Unfortunately, this approach often fails. Up to 20 percent of patients experience some type of gastrointestinal discomfort while taking 180 mg of elemental iron per day using this regimen17
, and 30 per cent of some patient groups may self discontinue the medication.18
Major stumbling blocks toward successful oral iron therapy are dose-related, upper gastrointestinal side effects such as nausea and epigastric discomfort which occur approximately one hour after ingestion. Lower gastrointestinal side effects such as constipation and diarrhea are less dose-related and are managed by symptom specific remedies (e.g. magnesium citrate for constipation).19
If a patient quickly becomes constipated or nauseated from commonly recommended dose of 150-200mg of daily elemental iron, dose reductions are applied. Changes in iron salts (and hence elemental iron per tablet) and formulations are commonly tried, and most involve dose reductions by lengthening the dose interval.19
These dose-lowering maneuvers may permit iron-intolerant patients to continue oral therapy, and avoid parenteral therapy.
In some centers, an outpatient measurement of oral iron absorption is performed for suspected malabsorption among iron deficient patients. A fasting serum iron level is compared to the level measured 1-2 hours after oral ingestion of 324 mg ferrous sulphate (66mg elemental iron). If the serum iron increases over 100 μg/dl from the baseline, iron absorption is likely adequate ().8
Despite the simplicity of iron absorption testing, the utility of this approach has been challenged.12,20
Along with consideration parenteral iron therapy, diagnosis and therapy for potentially reversible gastrointestinal diseases including autoimmune gastritis, Helicobacter pylori
and celiac disease must be considered for those patients with demonstrable malabsorption.21