Significant Interactions

Iron interacts with a variety of foods, herbs and drugs through several different mechanisms. Most commonly, the formation of insoluble complexes occurs whereby both iron and drug absorption is hindered. Separation of doses by several hours will often reduce the severity of this type of interaction. Additionally, substances that alter gastric pH have the theoretical ability to reduce iron absorption. A summary of interactions has been presented in table form for easy reference.

Drug/therapeutic substance

Mechanism

Possible outcome

Action required

ACE inhibitors

Reduced absorption of ACE inhibitors. A small clinical trial found that concomitant iron administration reduced area-under-the-curve plasma levels of unconjugated captopril by 37% (Lee et al 2001, Schaefer et al 1998)

Reduced drug effect

Separate doses by at least 2 hours

Antacids and products containing aluminium, calcium or magnesium

Reduces iron absorption (O'Neil-Cutting & Crosby 1986)

Reduced effect of iron

Separate doses by at least 2 hours

Ascorbic acid

Increases iron absorption

Increased effects of iron

Beneficial interaction possible— caution in haemochromatosis

Cholestyramine and colestipol

In vitro investigations have shown that cholestyramine and colestipol both bind iron citrate (Leonard et al 1979)

Reduced effect of iron

Monitor for iron efficacy if cholestyramine is being used concurrently

Separate doses by 4 hours. Increased iron intake may be required with long-term therapy

Cimetidine

Iron can bind cimetidine in the gastrointestinal tract and reduce its absorption (Campbell et al 1993)

Reduced effect of iron and drug

Separate doses by at least 2 hours

Dairy products and eggs

May reduce iron absorption

Reduced effect of iron

Monitor for iron efficacy

Erythropoietin

Pharmacodynamic interaction (Carnielli et al 1998)

Additive pharmacological effect possible

Beneficial interaction possible

Drug/therapeutic substance

Mechanism

Possible outcome

Action required

H2-receptor antagonists (antiulcer drugs)

Iron absorption is dependent upon gastric pH; therefore, medications that affect gastric pH may interfere with absorption of iron (Aymard et al 1988)

Reduced effect of iron

Monitor for iron efficacy if these drugs are being used concurrently

Haloperidol

May cause decreased blood levels of iron (Leenders et al 1994, Threlkeld 1998)

Reduced effect of iron

Monitor for iron efficacy if these drugs are being used concurrently Increased iron intake may be required with long-term therapy

L-dopa and carbidopa

May reduce bioavailability of carbidopa and L-dopa (van Woert 1977)

Reduced drug effect

Separate doses by 2 hours

Omeprazole and other proton-pump inhibitors

Reduced iron absorption due to changes in gastric pH

Reduced effect of iron

Monitor for iron efficacy if omeprazole is being used concurrently

Penicillamine

Reduced drug and iron absorption

Reduced drug and iron effect

Separate doses by at least 2 hours Sudden withdrawal of iron during penicillamine use has been associated with penacillamine toxicity and kidney damage (Harkness & Blake 1982) — caution

Quinolone antibiotics (e.g. norfloxacin)

Reduced drug absorption (Brouwers 1992)

Reduced drug effect

Take drug 2 hours before or 4-6 hours after iron dosing

Monitor patient for continued antibiotic efficacy

Sulfasalazine

May bind together, decreasing the absorption of both (Dukes & Duncan 1995)

Reduced drug and iron effect

Separate doses by at least 2 hours

Drug/therapeutic substance

Mechanism

Possible outcome

Action required

Tannins — herbs with significant tannin content (e.g. green tea, bilberry, raspberry leaf)

Tannin can bind to iron and reduce its absorption

Reduced effect of iron

Monitor for iron efficacy if these herbs are being used concurrently Separate doses by 2 hours

Tetracycline antibiotics (e.g. minocycline, doxycycline)

Reduced drug and iron absorption (Neuvonen 1976)

Reduced drug effect

Monitor for iron efficacy if tetracyclines are being used long term Separate doses by 4 hours

L-thyroxine

Decreased drug absorption possible. Iron supplements may decrease absorption of thyroid medication; however, iron deficiency may impair the body's ability to make thyroid hormones

Reduced drug effect

Thyroid function should be monitored and l-thyroxine dose may need alteration during treatment with iron Separate doses by at least 2-4 hours (Shakir et al 1997)

Vitamin A

Iron supplementation may cause a redistribution of retinol inducing vitamin A deficiency in infants with marginal vitamin A status (Wieringa et al 2003)

Redistribution of retinol

Iron supplementation in infants should be accompanied by measures to improve vitamin A status

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