What are iron pills made of and how do they work?

November 17, 2025 •

4 min read

According to the World Health Organization, iron deficiency is the most common nutritional deficiency worldwide, affecting billions of people. To combat this, oral iron supplements in the form of pills are widely used, but many people don't know what are iron pills made of beyond the active mineral.

Quick Summary

Iron pills are made from various forms of the mineral, including ferrous salts and ferric complexes, combined with inactive ingredients like binders, fillers, and coatings. The specific formulation impacts how the body absorbs the iron and the potential for side effects, such as constipation or stomach upset.

Key Points

Primary Active Ingredient: Iron pills contain different types of iron, most commonly ferrous salts (like ferrous sulfate, gluconate, or fumarate) or newer ferric complexes.
Enhanced Absorption: Chelated forms, such as ferrous bisglycinate, bind iron to amino acids for superior absorption and fewer side effects compared to traditional salts.
Inactive Components: Pills are held together and coated with inactive ingredients, including binders, fillers, lubricants (e.g., magnesium stearate), and colorants.
Formulation Matters: Enteric coatings or slow-release formulas can change where the pill dissolves in the digestive system, which can help minimize gastrointestinal issues.
Absorption Inhibitors: Certain foods and medications can significantly hinder iron absorption, including dairy products, coffee, and antacids, and should be avoided when taking iron pills.
Medical Consultation: A healthcare professional should determine the correct iron type and dosage, as factors like iron status, tolerance, and other health conditions affect the best choice.

Active Ingredients: The Different Types of Iron

At the core of every iron pill is the active ingredient, which delivers the essential mineral to the body. These can be divided into several categories, each with its own characteristics related to bioavailability, cost, and potential for side effects. The most common types are ferrous salts, often considered the gold standard for oral iron supplementation.

Ferrous Salts

Ferrous salts (Fe2+) are highly absorbable and widely available. The most frequently used types include:

Ferrous Sulfate: The most common and inexpensive form, containing approximately 20% elemental iron. It's effective but can cause significant gastrointestinal side effects.
Ferrous Gluconate: Often marketed as being gentler on the stomach than ferrous sulfate. It contains less elemental iron per tablet, meaning you might need to take more for the same dose.
Ferrous Fumarate: Contains a higher percentage of elemental iron per dose compared to ferrous sulfate or gluconate, making it a powerful option for treating severe deficiencies.

Ferric Complexes

Ferric complexes (Fe3+) are an alternative to ferrous salts. They are often more expensive but may be better tolerated by some individuals.

Ferric Citrate: A water-soluble compound that also functions as a phosphate binder, useful for patients with chronic kidney disease.
Ferric Maltol: A newer, lipophilic formulation of iron that has shown efficacy in treating iron deficiency anemia with fewer side effects than traditional salts.

Chelated and Other Forms

Advanced formulations aim to improve absorption and minimize side effects.

Ferrous Bisglycinate: A chelated form where iron is bound to amino acids, resulting in superior bioavailability and reduced stomach irritation.
Carbonyl Iron: A pure elemental iron that is slowly dissolved and absorbed in the stomach, which can reduce toxicity risks.
Heme Iron Polypeptide: Derived from bovine hemoglobin, this form is more easily absorbed than non-heme iron but can be more costly.

Inactive Ingredients: Binders, Fillers, and Coatings

Beyond the active iron compound, pills contain inactive ingredients (excipients) that give them their shape, consistency, and stability. These can vary significantly by brand.

Binders: Ingredients like microcrystalline cellulose, povidone, and starch hold the tablet together, preventing it from crumbling.
Fillers: Also known as diluents, fillers such as dicalcium phosphate or lactose are used to increase the size of the tablet, making the dosage easier to handle.
Lubricants: Magnesium stearate and stearic acid are common lubricants that prevent ingredients from sticking to manufacturing equipment during production.
Coatings: Many iron pills are film-coated to mask the metallic taste, protect the active ingredient from moisture, and sometimes to create an enteric-coated (slow-release) formulation. Common coating agents include hypromellose, titanium dioxide, and polyethylene glycol.
Colorants: Ingredients like FD&C Red #40, FD&C Blue #1, or caramel coloring are added for brand recognition and to distinguish different doses or products.

Comparison of Common Iron Formulations


Feature	Ferrous Sulfate	Ferrous Gluconate	Ferrous Bisglycinate
Cost	Low	Medium	High
Elemental Iron %	~20%	~12%	Varies, typically higher
Bioavailability	Good	Fair	Excellent (chelated)
Tolerability	Low (high GI side effects)	Medium (gentler)	High (low GI side effects)
Absorption Rate	Fast	Medium	Fast/Efficient
Recommended For	Standard deficiency treatment	Sensitive stomachs, maintenance	Sensitive stomachs, higher needs
Potential Issues	Stomach upset, constipation, dark stools	Less potent, still some GI risk	Higher cost, less common

Factors Affecting Iron Absorption

Numerous factors influence how effectively your body absorbs iron from a pill, a process that happens primarily in the duodenum of the small intestine.

Food and Drink: Taking iron with certain foods or beverages, like coffee, tea, or dairy products, can inhibit absorption. High-fiber foods and phytates can also interfere. Conversely, vitamin C-rich foods or supplements significantly enhance non-heme iron absorption.
Stomach Acidity: Sufficient stomach acid is essential for dissolving and absorbing most iron salts. Certain medications like antacids or proton pump inhibitors can reduce stomach acid and inhibit absorption.
Formulation Type: As noted in the table above, the type of iron can impact how easily the body absorbs it. Some advanced formulations are specifically designed for better absorption.
Timing: Taking an iron pill on an empty stomach is recommended for best absorption, though some people may need to take it with a small meal to reduce stomach upset. It's also best to space it at least two hours apart from other minerals like calcium, magnesium, or zinc.

The Role of Excipients in Manufacturing

Excipients are not just inert fillers; they are critical for the pill's stability, shelf life, and how it is released and absorbed in the body.

Disintegrants: Ingredients like croscarmellose sodium or sodium starch glycolate help the tablet break apart quickly in the digestive tract, ensuring the active iron is released for absorption.
Flavoring and Sweeteners: Some liquid or chewable iron supplements may contain sweeteners like sucrose or aspartame to improve taste.
Release Mechanism: Enteric-coated formulations use a protective layer to prevent the tablet from dissolving in the stomach, instead releasing the iron in the small intestine to minimize side effects. This is an intentional use of inactive ingredients to alter the drug's performance. More information on excipient functionality can be found on pharmaceutical websites like PharmaCompass, which details the roles of different ingredients like coatings and binders.

Conclusion

In summary, what iron pills are made of extends far beyond just the mineral itself. A pill's core composition includes an active iron compound, such as ferrous sulfate or a chelated form, combined with a variety of inactive ingredients that ensure its stability, a pleasant taste, and proper absorption. The optimal choice depends on a balance between efficacy, tolerability, and individual health needs, and often requires consulting a healthcare professional. While ferrous salts remain a common and effective option, newer formulations offer improved absorption and reduced side effects for those with sensitive digestive systems.

Frequently Asked Questions

Ferrous sulfate is the most common form of iron used in pills due to its effectiveness and low cost, although it is also known for causing significant gastrointestinal side effects like constipation.

Iron pills are typically manufactured by combining the active iron compound with inactive ingredients such as binders, fillers, and lubricants. The mixture is then compressed into tablets or put into capsules, sometimes with a film or enteric coating.

No, not all iron pills cause constipation. While it is a common side effect, especially with ferrous sulfate, some newer formulations like ferrous bisglycinate or certain slow-release versions are designed to be gentler on the stomach and reduce adverse effects.

Yes, timing can be important. Iron is best absorbed on an empty stomach, typically an hour before or two hours after a meal. Taking it with vitamin C can boost absorption, while dairy products, coffee, and tea can inhibit it.

It depends on the vitamin. You should avoid taking iron pills with calcium, zinc, and magnesium, as they can compete for absorption. However, iron supplements are often combined with vitamin C or B vitamins like folic acid and B12, which can support red blood cell production.

Chelated iron pills, such as ferrous bisglycinate, offer superior bioavailability and are typically better tolerated than traditional iron salts. This can result in fewer gastrointestinal side effects like stomach upset and constipation.

Enteric-coated iron pills have a special coating that prevents them from dissolving in the stomach's acidic environment. Instead, they dissolve later in the small intestine, which can help reduce stomach irritation and other gastrointestinal side effects.