If you have ever been prescribed iron and stopped taking it because of the side effects — the constipation, the nausea, the stomach cramps — you are not alone. Most people assume that is just what iron does. It is not. It is what a specific form of iron does. And there is a better option.
Iron deficiency is the most common nutritional deficiency in the world, and women are disproportionately affected. Yet the supplement most commonly recommended — iron sulphate — is also the one most likely to cause the side effects that stop people taking it. The result is that many women are prescribed iron, struggle with the side effects, and quietly stop without ever addressing the deficiency that led them there in the first place.
Understanding the difference between iron forms is not complicated — and it could make a significant difference to your experience of supplementing iron and the results you get from it.
The Problem With Iron Sulphate
Iron sulphate (also written as ferrous sulphate) is the most widely prescribed form of iron in Ireland and the UK. It is cheap, widely available, and the default recommendation from most GPs. It is also, for a significant proportion of people, very difficult to tolerate.
Why it causes problems
Iron sulphate releases iron ions rapidly in the digestive tract. This creates a high concentration of free iron in the gut — iron that has not yet been absorbed — which causes oxidative stress in the intestinal lining, irritates the gut wall, and disrupts the balance of the gut microbiome. The result for many people is:
- Constipation — often severe
- Nausea or stomach cramping
- Dark or black stools
- Heartburn or acid reflux
- Bloating and general digestive discomfort
These are not minor inconveniences. For many women they are significant enough to stop supplementation entirely — which means the iron deficiency remains unaddressed, often for months or years.
What Is Iron Bisglycinate?
Iron bisglycinate is a chelated form of iron — meaning the iron molecule has been bound to two molecules of the amino acid glycine. This binding process fundamentally changes how the iron behaves in the body.
Rather than releasing free iron ions into the gut — which cause the irritation and side effects associated with iron sulphate — iron bisglycinate is absorbed intact through a different transport mechanism, one that bypasses the conventional iron absorption pathway in the intestinal lining. This means:
- Significantly less free iron in the gut
- Less oxidative stress in the intestinal lining
- Far fewer digestive side effects
- Better absorption even when taken with food
- Less interference with the gut microbiome
Iron Bisglycinate vs Iron Sulphate — A Direct Comparison
| Factor | Iron Sulphate | Iron Bisglycinate |
|---|---|---|
| Form | Inorganic salt | Chelated amino acid complex |
| Absorption rate | Lower | Higher — roughly 2x |
| Digestive side effects | Common — constipation, nausea, cramping | Significantly reduced |
| Can be taken with food | Reduced absorption with food | Can be taken with food without significant loss |
| Effect on gut microbiome | Disrupts gut bacteria balance | Minimal disruption |
| Compliance | High discontinuation due to side effects | Significantly better tolerated |
| Cost | Lower | Higher — but often requires lower dose |
| Availability | Widely prescribed on prescription | Available over the counter and online |
Who Should Consider Iron Bisglycinate?
Iron bisglycinate is particularly worth considering if:
- You have tried iron sulphate and experienced significant side effects
- You have a sensitive digestive system or a history of IBS, constipation or gut inflammation
- You need to take your iron supplement with food (for example, if you experience nausea taking it on an empty stomach)
- You are taking iron as a preventive measure at a lower dose rather than treating a confirmed deficiency
- You are already dealing with digestive issues and don't want to add another stressor to your gut
How To Take Iron Bisglycinate For Best Results
Pair with Vitamin C
Vitamin C significantly enhances iron absorption by converting ferric iron to ferrous iron, which is more easily absorbed. Take your iron supplement alongside a food source of vitamin C — a glass of orange juice, a kiwi, or a handful of strawberries — or take a vitamin C supplement at the same time.
Avoid taking with calcium
Calcium and iron compete for the same absorption pathway. Avoid taking iron within two hours of any calcium supplement, dairy product, or antacid.
Avoid tea and coffee around the time of supplementation
The tannins in tea and coffee bind to iron and significantly reduce its absorption. Leave at least an hour gap between your iron supplement and your next cup.
Timing
Iron sulphate is typically recommended on an empty stomach to maximise absorption — but this often worsens side effects. Iron bisglycinate can be taken with a small amount of food without significantly compromising absorption, which makes it far more practical and comfortable.
The Bottom Line
Iron sulphate is the default because it is cheap and familiar — not because it is the best option. For the many women who have struggled with its side effects, iron bisglycinate offers a genuinely better alternative. Better absorbed, better tolerated, and far less likely to be abandoned before it has a chance to work.
The best supplement is the one you will actually take. For iron, bisglycinate makes that considerably more likely.
Not sure if you're iron deficient?
Take the free Mineral Questionnaire — 48 yes or no questions that help identify where your body may be falling short. Julie personally reviews every submission and follows up with tailored guidance within 48 hours.
Take the free Mineral Questionnaire →Julie Piper Roche is a Nutritional Therapist and Personal Trainer based in Ireland, and founder of The Supplement Coach. She holds a BSc (Hons) in Nutritional Therapy from the Institute for Optimum Nutrition (ION) and a Diploma in Nutritional Therapy from the IINH. The information in this article is for educational purposes only and does not constitute medical advice. Always consult your GP or a qualified healthcare practitioner before starting a supplement programme.