The Problem With Ferrous Sulphate - Side Effects, Poor Absorption & What to Take Instead

Ferrous sulphate is the most commonly prescribed iron supplement in Australia. It's also one of the most commonly abandoned with studies suggesting that up to 50% of patients stop taking it within the first few weeks due to gastrointestinal side effects.

If you've been prescribed ferrous sulphate and couldn't tolerate it - you didn't fail the treatment. The treatment failed you.

What is ferrous sulphate?

Ferrous sulphate is an inorganic iron salt, a simple compound of iron and sulphate that has been used as a medicinal iron supplement since the early 20th century. Its widespread use is largely a function of cost and historical familiarity, not because it represents the best available option.

Standard ferrous sulphate tablets contain 200-325mg per tablet, delivering approximately 60-65mg of elemental iron. The theoretical appeal is the high elemental iron content. The clinical reality is more complicated.

The absorption problem - what actually gets in

Bioavailability - the proportion of a nutrient that is absorbed and used by the body is the critical metric for any supplement. Elemental iron content tells you what's in the tablet. Bioavailability tells you what your body actually gets.

Ferrous sulphate has a bioavailability of approximately 10-15% under optimal conditions. Optimal conditions means taken on an empty stomach, with water, away from food, coffee, tea, calcium and other minerals.

In the real world, where supplements are taken with breakfast, alongside other medications, or with a morning coffee bioavailability drops further. Studies in free living populations show effective absorption rates of 5-10% are common.

This means a 200mg tablet might deliver as little as 10-20mg of usable iron. For a pregnant woman requiring 27mg of additional iron per day, the gap between what's in the tablet and what the body actually receives is significant.

Why the side effects are so predictable

The gastrointestinal side effects of ferrous sulphate are not random, they are a direct and predictable consequence of unabsorbed iron remaining in the gut.

When ferrous sulphate reaches the large intestine unabsorbed, it causes:

• Constipation: Free iron draws water from the intestinal mucosa, hardening stool
• Nausea and stomach cramps: Free ionic iron is directly irritating to the gut lining
• Black, tarry stools: Oxidised iron passing through the bowel
• Heartburn and reflux: Particularly when taken with food or lying down
• Metallic taste: Free iron interacting with salivary proteins
• Disruption to the gut microbiome: Unabsorbed iron feeds certain pathogenic bacteria preferentially

The conventional advice - 'take it with food to reduce nausea' simultaneously reduces bioavailability, creating a lose lose situation.

• Take it with food: more tolerable, less absorbed.
• Take it without food: better absorbed, harder to tolerate.

Iron bisglycinate (glycinate) - what the evidence shows

Iron bisglycinate (ferrous bisglycinate) is a chelated form of iron - meaning the iron molecule is bound to two glycine amino acids. This structural difference has clinically significant effects on both absorption and tolerability.

Improved bioavailability

Multiple comparative studies show iron bisglycinate absorbs at 2-4 times the rate of ferrous sulphate. A 2014 meta-analysis found chelated iron forms consistently outperformed inorganic iron salts for bioavailability across populations. This means the same haematological outcome can be achieved with a lower elemental iron dose.

Dramatically better tolerability

Because chelated iron is absorbed more efficiently and less free iron remains in the gut, the side effect profile is substantially different:

• Constipation rates are significantly lower - the most cited reason for individuals stopping ferrous sulphate
• Nausea rates are lower - chelated iron doesn't have the same direct irritant effect on the gut lining
• Can be taken with or without food with less impact on absorption
• More neutral effect on the gut microbiome

Less affected by dietary inhibitors

Chelated iron is less inhibited by common dietary factors - phytates, polyphenols, calcium than ferrous sulphate. This makes real world absorption more consistent and less dependent on strict timing around food and beverages.

Lactoferrin - supporting iron absorption and gut tolerance

Lactoferrin is a milk-derived glycoprotein with a distinct role in iron metabolism. Unlike iron supplements, lactoferrin doesn't deliver iron directly - it regulates how iron is absorbed and stored, with several clinically relevant effects:

• Enhances iron absorption by binding and transporting iron across the intestinal wall
• Has anti-inflammatory properties in the gut - relevant given that ferrous sulphate causes gut inflammation
• Supports healthy iron regulation without causing iron overload
• Has antimicrobial properties - relevant given the microbiome disruption caused by unabsorbed iron

Research combining iron glycinate with lactoferrin shows promising results for both efficacy and tolerability and this combination represents an emerging area of interest in pregnancy iron supplementation.

What to discuss with your care provider

If ferrous sulphate is not working for you - whether due to side effects or lack of efficacy, you have options. These are the specific conversations worth having at your next appointment:

• Ask about iron glycinate specifically
• Discuss dose splitting or alternate day dosing - lower doses twice daily often improve tolerability with ferrous sulphate, though a form switch is usually more effective
  
• Request a full iron study to guide treatment, not just haemoglobin. Ferritin, serum iron, transferrin saturation, and CRP all contribute to a complete picture

Frequently asked questions

1. Is constipation from iron supplements inevitable? Not with the right form of iron. Constipation is strongly associated with ferrous sulphate due to the high proportion that remains unabsorbed in the gut. Chelated iron forms. particularly iron glycinate - cause constipation at significantly lower rates.

2. Can I take iron without a prescription in Australia? Low-dose iron supplements are available over the counter in Australia. Higher dose iron supplements and specific forms may require a pharmacist consultation or prescription. Speak with your GP or pharmacist about the most appropriate option for your ferritin level and pregnancy stage.

3. Does taking iron with vitamin C actually help? Yes, vitamin C significantly enhances non-haem iron absorption by reducing ferric iron (Fe³⁺) to the more absorbable ferrous form (Fe²⁺). This is most relevant for dietary non-haem iron and for ferrous sulphate.

Related reading

→ Part 1: Why Women Enter Pregnancy Iron Depleted 

→ Part 3: Reading Your Iron Blood Test