What are the side effects of taking iron supplements?

Starting a new supplement routine often brings a mix of hope and hesitation. You want to feel better. However, you might also worry about how your body will react. This is a completely natural concern.

It’s important to know that most people take iron supplements without any serious issues. Side effects are typically mild and they often disappear as your body adjusts. 

However, everyone is unique. If you feel unwell or notice side effects different from those described here, you should speak to your doctor or pharmacist. They can offer personalised advice. This might involve adjusting your dosage or switching to a gentler formulation.

Below, we discuss some frequently-asked questions relating to iron supplement side effects.

Do iron supplements make you constipated?

This is the most common question regarding iron intake. For many, the answer is yes. Gastrointestinal (GI) discomfort is the primary side effect associated with oral iron therapy, though it is usually mild.¹

Why does this happen? Well, iron is one of the most difficult nutrients for the body to absorb. Any iron that isn’t absorbed remains in the gut, and this can irritate the stomach lining and alter bowel movements. The result is often constipation.

However, “GI discomfort” is a broad term that covers more than just constipation. You might experience a range of symptoms that affect the digestive tract.

Nausea and Stomach Pain

Some people experience nausea shortly after taking their tablet. This is often accompanied by stomach cramps or bloating, which happens because iron salts can be harsh on the stomach lining.¹ 

Taking the supplement on an empty stomach maximises absorption. Unfortunately, it can also increase the risk of nausea. If this is the case for you, try taking the supplement with food to see if this makes you feel better. 

Dark Stools

You might notice your stools turning very dark or even black while you’re taking iron supplements. This can look alarming, but it’s usually harmless. The odd colouring is simply the result of unabsorbed iron passing through your system – it’s a sign that you’re getting enough iron and doesn’t typically mean there’s anything wrong.² 

However, you should always mention changes in your bowel movements like this to your doctor, just to be on the safe side.

Managing these effects

If these GI symptoms become bothersome, doctors often suggest taking the supplement with food. This might slightly reduce absorption, but it can significantly reduce nausea and cramping, so it may be preferable.² Another strategy is staying well-hydrated and increasing fibre intake to combat constipation.

Research indicates that the severity of these side effects is often related to the dosage. Higher doses tend to cause more tummy trouble.¹ That’s why it’s crucial to never exceed your prescribed amount. 

If the discomfort continues after you’ve taken measures to alleviate the side effects, your healthcare provider might suggest a different form of iron that is gentler on the stomach.

Can iron supplements make you gain weight?

There is a common myth that iron supplements cause weight gain. In reality, iron itself contains zero calories. It does not directly increase body fat.

However, there is a biological link between iron levels and appetite. Some of the most common symptoms of iron deficiency are fatigue and a loss of appetite. As your iron levels recover through supplementation, your energy returns. Your natural appetite may also come back.³

This is a positive sign – t means your body is functioning better. So, while you may gain weight after starting iron supplements, it’s not usually something to worry about. In most cases, it simply means your body is returning to a healthy appetite.

Furthermore, some studies suggest that low iron levels can actually impair the metabolism, affecting your body’s ability to break down nutrients from your food.⁴ Correcting the deficiency can help your metabolism to work efficiently again. Therefore, any changes in weight are likely due to lifestyle and dietary choices rather than the mineral itself.

However, if you do notice a sudden increase or decrease in weight that doesn’t seem right, speak to your doctor or a pharmacist. They can help you to understand potential causes and advise you on what to do.

Do iron supplements make you tired?

It might seem like a contradiction – after all, many people take iron to treat fatigue, not to cause it. So why do some people worry that iron supplements can make you feel tired?

The first thing to understand is that iron is crucial for creating haemoglobin. This protein carries oxygen in your blood, and without enough of it, you feel exhausted.⁵ Taking iron supplements helps to avoid this. 

However, it takes time for the full effects of your iron supplements to reveal themselves. It can take weeks or sometimes even months to rebuild iron stores and generate new red blood cells. If you’ve only started supplements recently, it’s likely that the tiredness you feel is nothing more than a lingering symptom of the deficiency you’re trying to treat. 

In very rare cases, extreme fatigue could be a sign of iron overload (toxicity caused by taking too much iron). This is why you should only take the recommended dosage, and avoid taking iron supplements without a doctor’s advice. If your iron levels were fine beforehand, you may be at risk of raising them too high.

 

Can iron supplements give you headaches?

Headaches are a less common side effect, but they do occur. Some people report mild headaches when they first start taking iron supplements.

The exact mechanism behind this side effect isn’t yet fully understood. It may be related to changes in blood flow caused by the production of haemoglobin – or it might be another symptom of your body’s adjustment period. 

Like GI issues, these headaches are often short-lived. They tend to become less common and even go away altogether as your system gets used to the increased iron intake.²

If you experience sudden, severe headaches, do not ignore them. Please consult your doctor promptly to rule out other causes. For most people, however, staying hydrated and resting is enough to manage this temporary adjustment side effect.

Making Iron Work for You

Iron supplements can be a powerful tool for health. While it can cause mild side effects like constipation or nausea, these are generally manageable. They are rarely dangerous. Listening to your body and working with your GP ensures you get the benefits without unnecessary discomfort.

References

1. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. Tolkien, Z., et al. PLoS ONE, 2015.
2. Iron supplements: Guidance for patients. Mayo Clinic.
3. Common questions about iron supplements. National Health Service (NHS) UK.
4. Iron: Fact Sheet for Health Professionals. National Institutes of Health (NIH), Office of Dietary Supplements.
5. Iron-Deficiency Anemia. American Society of Hematology.