Ferrous sulfate is the most commonly prescribed oral iron supplement, used primarily to treat and prevent iron deficiency anemia. It delivers iron in a form your body can absorb, and a standard 325 mg tablet contains about 65 mg of elemental iron, the portion your body actually uses. It’s inexpensive, widely available over the counter, and has decades of clinical use behind it.
How Ferrous Sulfate Works in Your Body
Iron exists in two chemical states, and your intestines can only absorb one of them. Ferrous sulfate delivers iron in that absorbable state (called ferrous, or Fe2+), which is why it’s effective. Absorption happens mainly in the first stretch of your small intestine, just past the stomach, where specialized cells pull the iron into your bloodstream.
Once absorbed, about 70% of the iron in your body binds to hemoglobin, the protein in red blood cells that carries oxygen from your lungs to every tissue. Iron also plays roles in energy production and the synthesis of DNA and proteins. When your iron stores run low, your body can’t produce enough healthy red blood cells, and the result is iron deficiency anemia: fatigue, weakness, shortness of breath, and sometimes pale skin or dizziness.
Who Needs It
Iron deficiency anemia is the most common nutritional deficiency worldwide. Your doctor may check your ferritin level, a blood marker that reflects how much iron your body has stored. A ferritin level below 30 ng/mL strongly suggests iron deficiency, while levels at or above 100 ng/mL generally rule it out. If you also have chronic inflammation from conditions like autoimmune disease or kidney disease, iron deficiency is likely when ferritin drops below 50 ng/mL.
Groups at higher risk include women with heavy menstrual periods, pregnant women, young children during rapid growth phases, vegetarians, frequent blood donors, and people with digestive conditions that impair absorption (like celiac disease or inflammatory bowel disease).
During pregnancy, ferrous sulfate is one of the most frequently recommended supplements. The World Health Organization advises 30 to 60 mg of elemental iron daily for pregnant women to help prevent anemia, low birth weight, and preterm birth. That’s roughly equivalent to one 325 mg ferrous sulfate tablet per day. Folic acid, ideally started before conception, is typically recommended alongside it.
Standard Dosing
A typical over-the-counter ferrous sulfate tablet is 325 mg, but not all of that is iron your body uses. The hydrated form, which is the most common, contains about 20% elemental iron, giving you roughly 65 mg of actual iron per tablet. A desiccated (dried) version of the same 325 mg tablet is more concentrated, delivering about 120 mg of elemental iron, so it matters which formulation you’re taking.
For adults, the standard suggestion on most labels is one tablet daily, taken preferably after a meal. Your doctor may adjust this depending on how severe your deficiency is. Children’s dosing is weight-based and should always be guided by a pediatrician.
What Affects Absorption
How much iron actually makes it into your bloodstream depends heavily on what else is in your stomach at the time. Vitamin C is the single best absorption enhancer. It keeps iron in the form your intestines can absorb and helps shuttle it across intestinal cells. Taking your iron tablet with a glass of orange juice or alongside vitamin C-rich foods like bell peppers or strawberries can meaningfully increase how much iron you get from each dose.
On the other hand, several common substances block iron absorption. Calcium, whether from dairy or supplements, competes directly with iron for absorption. Tannins in tea and coffee, phytates in whole grains and legumes, and polyphenols in certain vegetables all reduce how much iron gets through. Antacids and proton pump inhibitors (heartburn medications) lower stomach acid, which your body needs to process iron properly. If you take any of these, spacing them at least two hours apart from your iron supplement helps.
Side Effects
Gastrointestinal problems are the main reason people stop taking ferrous sulfate. A large systematic review found that about 11% of adults taking ferrous sulfate experience nausea. Constipation, stomach cramps, and dark or black stools are also common. The dark stools are harmless, just a result of unabsorbed iron passing through, but they can be alarming if you’re not expecting them.
A few strategies can reduce these side effects. Taking the tablet with food lowers absorption slightly but often makes it much more tolerable. Some people do better splitting a dose or taking it every other day rather than daily, a dosing pattern that some newer research supports for maintaining absorption while reducing gut irritation. Liquid formulations may be easier on the stomach for people who struggle with tablets.
How Quickly It Works
If you’re taking ferrous sulfate for anemia, expect a gradual improvement rather than an overnight fix. A good response looks like a hemoglobin increase of about 2 g/dL within three to four weeks, though a rise of 1 g/dL in that time frame is still considered reasonable. You may start feeling less fatigued within the first couple of weeks as your body begins producing healthier red blood cells.
Replenishing your body’s deeper iron stores takes longer. Most people need to continue supplementation for several months after their hemoglobin normalizes, often until ferritin levels climb above 100 ng/mL. Stopping too early is one of the most common mistakes, because hemoglobin can bounce back while your reserves are still depleted, setting you up for a relapse.
How It Compares to Other Iron Supplements
Ferrous sulfate isn’t the only iron supplement on pharmacy shelves. Ferrous fumarate and ferrous gluconate are two common alternatives. Fumarate contains a higher percentage of elemental iron per milligram, while gluconate contains less. For context, 300 mg of ferrous sulfate, 180 mg of ferrous fumarate, and 500 mg of ferrous gluconate all deliver roughly the same 60 mg of elemental iron.
In practice, all three ferrous salts are absorbed similarly because they all deliver iron in the same usable form. The main differences come down to pill size, cost, and individual tolerance. Some people who get nauseous on ferrous sulfate find they tolerate gluconate better, likely because the lower elemental iron per dose is gentler on the stomach. Ferrous sulfate remains the most studied and most widely recommended simply because it’s effective, cheap, and available everywhere.
Iron Toxicity Risk
Iron supplements, including ferrous sulfate, are a leading cause of poisoning in young children. Ingesting more than 60 mg per kilogram of body weight is associated with serious toxicity, and the lethal dose is estimated at 200 to 250 mg per kilogram. For a small child, even a handful of adult-strength tablets can be dangerous.
Acute iron poisoning unfolds in distinct stages. In the first six hours, vomiting, abdominal pain, and lethargy dominate. This can be followed by a deceptive period of apparent improvement between 6 and 12 hours, which gives way to severe complications like liver failure, kidney damage, and shock between 12 and 48 hours. Survivors of severe poisoning may develop scarring in the stomach weeks later. Iron tablets should always be stored in child-resistant containers and kept well out of reach.