Ferrous Gluconate vs Ferrous Sulfate: 5 Key Variations to Know

While you can somewhat replenish your iron levels by eating iron-rich foods (e.g., spinach, red meat, or whole wheat bread), most people who have a true deficiency will need to take iron supplements, most often in the form of iron pills. But choosing the right type of iron—whether it’s ferrous gluconate vs. ferrous sulfate—can be confusing.

For starters, not all iron supplements are created equal: Choosing a pill that’s not right for your specific needs could result in unpleasant side effects (like an upset stomach or constipation) or simply not getting the boost in iron you need. Plus, while both ferrous gluconate and ferrous sulfate are the most popular iron options on the market, there are key differences between the two to note.

Here, we tapped a primary-care physician and a registered dietitian to create the ultimate guide on ferrous gluconate vs. ferrous sulfate, including the pros and cons of each and which is best to take for a deficiency.

5 differences between ferrous gluconate vs. ferrous sulfate

From how much elemental iron each has to how effective they are at treating deficiencies, here are five differences between ferrous gluconate vs. ferrous sulfate to know before trying them out.

1. Ferrous sulfate has more elemental iron

Iron supplements with ferrous sulfate have more elemental iron than ferrous gluconate, which basically refers to the pure form of iron. “Ferrous sulfate contains about 20 percent elemental iron, so a 325-milligram (mg) tablet has around 65 mg of elemental iron,” Amber Wheeler, MD, an endocrinologist and advisor for Hey Freya, tells Well+Good. In contrast, “ferrous gluconate contains about 12 percent elemental iron, meaning that a 325-mg tablet offers about 38 mg of elemental iron.”

Dr. Wheeler also notes that because iron supplements can cause digestive issues, a smaller dose of elemental iron is sometimes recommended to lower the risk of symptoms like nausea and constipation.

2. Ferrous gluconate may be better absorbed by your body

While there’s not much current research on this, older hallmark studies (like this 1996 clinical trial from Minerva Ginecologica) show that ferrous gluconate might be better absorbed in the body than ferrous sulfate, especially when taken in liquid form.

That said, “it does contain less elemental iron than ferrous sulfate, so more of the supplement may be needed,” says Meggie Connelly, MS, RD, LDN, a registered dietitian and owner of Be Balanced Nutrition Services.

3. Ferrous gluconate may be better for deficiencies

While there’s not much research showing the benefit for adults, “research into the absorption of ferrous gluconate vs. ferrous sulfate in toddlers shows that both are effective at preventing iron deficiency anemia,” says Connelly. More specifically, “ferrous gluconate increased hemoglobin and ferritin (a blood protein that contains iron) more than the individuals who supplemented with ferrous sulfate.”

Why is this important? As we’ve learned, iron is a key component of hemoglobin—the protein that helps carry oxygen throughout the body. The iron in hemoglobin binds to oxygen, allowing it to be transported to all your major organ systems. The ferritin is also important, as it helps store iron in the body and acts as a reserve to make sure there’s enough available when needed.

This balance is crucial because, without enough stored iron (ferritin), the body might not be able to make enough hemoglobin, which can lead to iron deficiency anemia. Because ferrous gluconate has been found to increase hemoglobin and ferritin levels more than ferrous sulfate in small clinical trials in children, it’s possible that it might be better at preventing and treating iron deficiencies.

4. Ferrous gluconate may be a better choice during pregnancy

The American Society for Nutrition notes that when you are pregnant, your iron needs increase tenfold. This is why it’s important to keep your iron levels strong throughout pregnancy with iron-rich foods and supplements.

While both types of iron supplements can increase your levels and decrease the risk of anemia during pregnancy, Connelly says ferrous gluconate may be a better choice when it comes to absorption and tolerance. But “it does have less elemental iron, so more (milligrams) of it may be needed,” she adds. It’s also often more expensive than ferrous sulfate, which can be a barrier for some pregnant people.

For those who are expecting, Connelly also recommends ferrous bisglycinate, which has been shown to increase iron levels better⁴ than other forms of iron. She says it tends to cause fewer side effects in pregnant people—like constipation and nausea.

5. Ferrous gluconate is easier on the stomach

“Ferrous gluconate is a bit better for the stomach due to having less elemental iron,” says Connelly. That said, if you’re not tolerating your current supplement, she recommends trying one with ferrous bisglycinate, which studies have found is absorbed well and gentler on the stomach⁴.

That said, some research (like this 2013 review in Current Medical Research and Opinion), has found that extended-release ferrous sulfate with mucoproteins (a protein that binds to iron and helps protect the stomach lining and reduce the risk of GI effects) is better tolerated than other forms. However, it’s typically poorly absorbed and not recommended for treating an iron deficiency.

How to take iron supplements so they’re most effective

For best absorption, Connelly suggests taking iron pills about 30 minutes before a meal. “Taking your iron with a good source of vitamin C⁶, like a supplement or orange juice, can also help with absorption,” she adds.

Even though it’s best to take iron on an empty stomach, Connelly knows that can exacerbate digestive woes. If this is the case for you “it’s okay to take your iron supplement with a small amount of food,” she says. Just be sure to avoid high-calcium foods (like dairy), high-fiber foods, and caffeine around when you take the supplement, as they can all impair iron absorption.

As far as how often to take iron supplements? A 2019 study found that for people with iron-deficiency anemia, pills were better absorbed when taken every other day⁷ instead of daily. (For context, this particular study, which used ferrous sulfate, found that taking iron supplements every other day led to significantly higher iron absorption than those taking it daily.)

And what about dosage? Dr. Wheeler says the U.S. Recommended Dietary Allowance (RDA) of elemental iron for healthy adults ranges from 7 to 27 milligrams per day, depending on your age and factors like pregnancy and lactation.

Some good supplements to try include Walgreen’s High-Potency Iron Ferrous Gluconate ($9) or NatureMade Iron Ferrous Sulfate ($13). And if your doctor thinks iron bisglycinate is best for you, look for Thorne Iron Bisgylcinate tablets ($13).

Side effects of taking iron supplements

Just like there are different types of iron supplements, there are two main types of iron⁸, too: heme and non-heme. Heme iron comes from animal sources and is typically better absorbed, while non-heme iron comes mostly from plant sources.

“First-line iron supplements are usually non-heme sourced and can cause several side effects, particularly if the dose is above the RDA,” says Dr. Wheeler. Some of the most common side effects of taking iron supplements include the following:

• Constipation
• Nausea
• Vomiting
• Diarrhea
• Gas
• Bloating
• Metallic taste
• Black and tarry stools

“Taking slow-release or heme iron supplements can help reduce the gastrointestinal side effects,” says Dr. Wheeler.

Speaking with your healthcare provider before starting iron supplements is always important, as too much iron can be dangerous. There’s also the matter of hemochromatosis—a genetic condition that causes the body to absorb too much iron from the food you eat, which can lead to serious health issues and damage the liver, heart, and pancreas over time. “Most people don’t realize that they have it until symptoms appear later in life, usually after age 50,” says Dr. Wheeler.

Because the symptoms of hemochromatosis can be non-specific (such as fatigue, joint pain, abdominal pain, “bronzed” skin, hair loss, and low sex drive), it’s important to have lab work related to iron levels done before starting supplements to make sure you don’t have hemochromatosis.

How long does it take to treat an iron deficiency?

“Reversing an iron deficiency takes time, and the duration depends on whether you’re dealing with an iron deficiency or iron deficiency anemia,” says Dr. Wheeler. With iron deficiency, the body’s low iron stores haven’t yet affected red blood cell production. She says symptoms like mild fatigue might go unnoticed, and iron lab tests often reveal the issue. “In this case, iron supplements typically take three to six months to fully replenish iron stores, but symptoms can often resolve in as little as two to three weeks,” she adds.

In contrast, with iron deficiency anemia, the iron stores have dropped so low that the body can’t produce enough hemoglobin to replace red blood cells, leading to more noticeable symptoms like fatigue, shortness of breath, hair loss, brittle nails, lightheadedness, and cold intolerance, says Dr. Wheeler.

“While symptoms may improve within a few days of iron supplementation, it takes at least six months of continued supplementation after blood counts return to normal to resolve iron deficiency anemia.” Plus, your healthcare provider will want to identify and understand the underlying causes if the iron deficiency is not linked to an obvious factor, like blood loss.

When to see the healthcare provider about low iron

According to Dr. Wheeler, iron deficiency can happen for many reasons, including decreased dietary intake, reduced absorption, or blood loss. It’s important to see a healthcare provider if you have any of the following symptoms, as they could indicate an iron deficiency:

• Persistent fatigue
• Pica (unusual cravings for non-food items like ice or dirt)
• Pale skin
• Shortness of breath

Dr. Wheeler also notes that certain people are more likely to become iron-deficient, including:

• People with heavy menstrual bleeding
• People following a vegan or vegetarian diet
• People who are pregnant or lactating
• Frequent blood donors
• Older adults with kidney or heart conditions
• Individuals who lack access to nutritious foods

“Your doctor may recommend blood tests based on your risk factors, medical history, or medications, and while not everyone requires routine screenings, regular blood work is crucial for those at risk, especially if you’re not responding to iron supplements, to ensure timely treatment,” says Dr. Wheeler.

The takeaway

When it comes to ferrous gluconate vs. ferrous sulfate, the consensus seems to be that ferrous gluconate is a better choice for treating deficiencies. Plus, due to its lower elemental iron content, it may be less likely to cause GI side effects. That said, iron supplementation should be tailored to your individual needs, and it’s important to see a healthcare provider to get screened for iron deficiency before starting any supplements.