Iron is the most common supplement for anemia, but it’s not always the right one. The best supplement depends on what’s causing your anemia: iron deficiency, vitamin B12 deficiency, folate deficiency, or sometimes a combination. Getting the type wrong means months of taking pills that won’t help. Here’s what works, how to take it effectively, and what else your body needs to actually use it.
Iron: The Most Common Fix
Iron deficiency is the leading cause of anemia worldwide. Your doctor can confirm it with a blood test measuring ferritin, a protein that stores iron. The World Health Organization defines iron deficiency as ferritin below 15 μg/L for women and below 12 μg/L for young children, though newer research suggests hemoglobin starts declining at ferritin levels around 23 to 25 μg/L, well above those official cutoffs.
Most over-the-counter iron supplements come as ferrous sulfate, ferrous gluconate, or ferrous fumarate. All work. The tolerable upper intake level for adults is 45 mg of elemental iron per day from supplements, according to the NIH. Going far beyond that gets dangerous: a single acute dose above 20 mg per kilogram of body weight (roughly 1,365 mg for a 150-pound person) can cause serious intestinal damage, shock, and organ failure.
Why Every Other Day Beats Daily Dosing
One of the most useful things to know about iron supplements is that taking them every other day can actually improve how much iron your body absorbs. When you take an iron dose, your liver releases a hormone called hepcidin that spikes for about 24 hours and temporarily blocks further iron absorption. In a crossover study of women with iron deficiency anemia, iron absorption on alternate days was 40 to 50% higher than on consecutive days, because the hepcidin surge had time to clear before the next dose.
If your target is 100 mg daily, you can take 200 mg every other day instead and absorb roughly twice as much total iron. This approach also tends to cause fewer side effects, which is a major practical benefit since nausea, constipation, and stomach upset are the top reasons people quit their iron supplements.
Reducing Side Effects
Constipation and diarrhea are the most common complaints. Nausea and vomiting tend to happen at higher doses but often improve if you split the dose into smaller amounts. If constipation becomes a real problem, a stool softener like docusate sodium can help. And if one form of iron bothers your stomach, switching to a different form (say, from ferrous sulfate to ferrous gluconate) is worth trying before giving up entirely.
What Blocks Iron Absorption
Certain foods and drinks dramatically reduce how much iron your body takes in. Polyphenols, found in tea, coffee, and red wine, are potent blockers. In one study, just 50 mg of polyphenols lowered iron absorption by 14%, while 200 mg cut it by 45%. Phytic acid, concentrated in beans, whole grains, and nuts, has a similar effect. When researchers removed both polyphenols and phytic acid from bean porridge, absorption jumped 2.6-fold.
Calcium also competes with iron for absorption. The practical rule: take your iron supplement on an empty stomach, or at least avoid coffee, tea, dairy, and high-fiber grains within an hour or two of your dose.
Does Vitamin C Help?
Vitamin C has long been recommended alongside iron supplements. It creates a more acidic stomach environment and helps keep iron in its more absorbable form. However, a randomized clinical trial published in JAMA Network Open found that adding 200 mg of vitamin C to each iron dose three times daily for three months produced no meaningful improvement in hemoglobin or iron absorption compared to iron supplements alone.
Vitamin C remains the only dietary factor besides animal tissue shown to promote iron absorption, so eating citrus or peppers with iron-rich meals still makes sense. But buying a separate vitamin C supplement specifically to pair with your iron pill may not be necessary.
Vitamin B12 for a Different Kind of Anemia
Not all anemia comes from low iron. Vitamin B12 deficiency causes megaloblastic anemia, where your red blood cells grow abnormally large and can’t carry oxygen efficiently. This type is especially common in people over 50 (who often absorb B12 poorly from food), vegans and strict vegetarians, and anyone with digestive conditions affecting the stomach or small intestine.
For B12 deficiency caused by poor dietary intake or impaired food absorption, oral supplementation at 1,000 to 2,000 μg per day for one month, followed by a maintenance dose of 125 to 500 μg daily, is considered safe and effective. People with pernicious anemia, an autoimmune condition that destroys the cells needed to absorb B12, traditionally receive injections, though high-dose oral supplements can also work because a small percentage of B12 absorbs passively without those cells.
Folate: Often Needed Alongside B12
Folate deficiency produces the same type of oversized, dysfunctional red blood cells as B12 deficiency, and the two often overlap. Because measuring folate levels in the blood is unreliable, experts recommend that anyone being treated for B12 deficiency also take folate supplementation at 400 μg to 1 mg per day. This is important because folate can temporarily mask B12 deficiency symptoms while neurological damage quietly progresses, so both should be addressed together rather than folate alone.
Copper: The Overlooked Cofactor
Copper plays a behind-the-scenes role that most people never hear about. Your body needs copper-dependent enzymes to move iron out of intestinal cells and load it onto transport proteins in your blood. Without enough copper, iron can get trapped in your gut lining, and your bone marrow can’t efficiently build the oxygen-carrying molecule in red blood cells.
Copper deficiency mimics iron deficiency anemia closely: low hemoglobin, low serum iron. It’s rare in people eating a varied diet but can occur with excessive zinc supplementation (zinc and copper compete for absorption), after gastric surgery, or with prolonged use of denture adhesives containing zinc. If iron supplements aren’t improving your numbers after several weeks, copper status is worth investigating.
Higher Needs for Athletes, Vegans, and Pregnancy
The standard recommended daily allowance for iron is 18 mg for adult women in the U.S. and 14.8 mg in the UK. Pregnancy pushes that requirement significantly higher because of expanded blood volume and fetal development. Prenatal vitamins typically include extra iron for this reason.
Female endurance athletes, particularly distance runners, may need roughly 70% more iron than the general recommendation. The combination of foot-strike red blood cell damage, sweat losses, and exercise-related inflammation increases iron turnover in ways that diet alone struggles to cover.
Vegans and vegetarians face a different challenge. Plant-based iron (non-heme iron) is absorbed far less efficiently than the heme iron found in meat. In one study comparing vegetarian and omnivore runners with similar total iron intakes, the vegetarians had dramatically lower ferritin levels (7.4 μg/L versus 19.8 μg/L) because they absorbed only about two-thirds as much iron from their food. If you eat a plant-based diet, getting your ferritin checked periodically is a smart move, even if your dietary iron intake looks adequate on paper.
Matching the Supplement to the Cause
The single most important step is identifying why you’re anemic before choosing a supplement. Iron won’t fix B12 deficiency. Folate won’t fix iron deficiency. Taking high-dose iron when the real problem is copper deficiency means months of side effects with no improvement. A basic blood panel measuring hemoglobin, ferritin, B12, and folate levels will point you in the right direction, and most of the effective supplements are inexpensive and widely available once you know which one you actually need.