Yes, iron pills often cause constipation. In trials of ferrous sulfate, a common iron pill, about 12% of people reported it. Start with more fluids, fiber added slowly, daily movement, and taking iron with a meal if your label allows it. If that isn't enough, ask your prescriber about a different form. Every-other-day dosing cut stomach pain in a trial, but not clearly constipation.
That 12% comes from a 2015 meta-analysis of 43 trials in PLoS One. Constipation (12%) was about as common as nausea (11%) and more common than diarrhea (8%). So yes, this is normal. It's also fixable, and quitting your iron on your own usually isn't the fix.
This article provides general information about medication management and is not a substitute for professional medical advice. Consult your doctor or pharmacist for advice specific to your medications.
Why Do Iron Pills Cause Constipation?
Your body absorbs only part of each iron pill. The rest moves on through your gut. As the team behind a 2023 iron trial put it, "unabsorbed iron in the gut can trigger gastrointestinal (GI) symptoms," per the trial report in eClinicalMedicine.
One ferrous sulfate label on DailyMed says it directly too: "Iron containing products may cause constipation and diarrhea," per DailyMed. Same pill, opposite problems, depending on the person.
The same 43-trial analysis also found "no significant association between dose and gastrointestinal side-effects." So "just take less" is a weaker fix than it sounds. One pregnancy study did see more constipation at its highest dose, so the evidence is mixed. Dose is your prescriber's call.
Constipation From Iron Pills: What to Do Now
None of these change your prescription.
- Drink more. The NIH's digestive disease institute (NIDDK) says to drink plenty of water and other liquids, especially when you add fiber.
- Add fiber slowly. Adults need about 22 to 34 grams a day, depending on age and sex, per NIDDK. Our high-fiber foods chart can help. Add a little each day. If you use a fiber supplement, ask your pharmacist whether to space it from your iron.
- Get moving. NIDDK lists being more active alongside diet and fluids.
- Take it with a meal, if your label allows. The ferrous sulfate label directs "1 tablet daily preferably with a meal or as directed by a doctor," per DailyMed. If your doctor said to take it on an empty stomach, ask before you switch. See our guide to the best time to take iron and iron and coffee.
- Still stuck? Ask your pharmacist about an over-the-counter option. NIDDK lists several types and says "you should only use stimulants if your constipation is severe or other treatments have not worked." Your pharmacist can match one to your other medicines.
Iron Every Other Day vs Daily: Does It Help Constipation?
It's the most common advice online. The best trial on it is more careful.
In 2023, Swiss researchers ran a double-blind, placebo-controlled trial with 150 young women (ages 18 to 45) who had low iron stores. One group took iron every day for three months. The other took the same total amount every other day over six months. Everyone logged side effects daily in a phone app.
On the days women actually took iron, the daily group reported gut side effects (all combined) 1.56 times as often. Over the full six months, with the same total iron, here's how the daily group compared with the every-other-day group:
| Side effect | Daily vs every other day (6 months) | What it means |
|---|---|---|
| Stomach pain | 1.56x as often | A clear difference |
| Any gut side effect | 1.20x as often | A clear difference, mostly from stomach pain |
| Nausea | 1.10x as often | Too small to rule out chance |
| Diarrhea | 1.06x as often | Too small to rule out chance |
| Constipation | 1.10x as often | Too small to rule out chance |
The numbers come from the trial tables. Over the full six months, only stomach pain was clearly more common on daily iron. The nausea, diarrhea, and constipation gaps were small enough to be chance. In the months women were taking iron, constipation's gap was always smaller than stomach pain's.
So every-other-day iron did make the stomach calmer. For constipation specifically, the trial can't promise much.
Every-other-day iron has a second selling point: absorption. In a 2017 trial in The Lancet Haematology, 40 women with low iron absorbed 21.8% of their iron on alternate days versus 16.3% on back-to-back days. A 2020 study of 19 women with anemia found similar results. The reason is in what to do if you forgot your iron supplement.
All of these trials were small and in young women. That's why the schedule is your prescriber's call. Your blood work matters here as much as your bowels do.
Iron Pills That Don't Cause Constipation: What the Evidence Says
We found no iron pill shown in trials to be constipation-free. Some forms do have better evidence than others.
Start with ferrous bisglycinate, which has been tested head to head against other forms. A 2023 review in Nutrition Reviews looked at 17 trials. In the trials in pregnant women, ferrous bisglycinate had about a third the rate of reported gut side effects of other iron pills. That counts all gut symptoms, not just constipation. Other adults were barely studied. A 2024 pregnancy study also found it the mildest of three forms, but it was given at a lower iron amount, so part of that gap may just be less iron.
Slow-release iron doesn't look like the answer either. In the 6 trials in that analysis that used it, gut side effects were not clearly lower than with regular ferrous sulfate.
Then there's a pill with less iron in it. It may feel gentler, but it can refill your stores more slowly. Our guide on whether 65 mg of iron is too much explains the numbers on your bottle.
If your gut can't handle any pill, tell your doctor. Iron given by IV caused far fewer gut side effects than ferrous sulfate in trials, though IV iron is given in a clinic and has its own risks. Your doctor can talk through the options.
When Constipation Isn't Just the Iron
NIDDK lists iron supplements among the medicines and supplements that can make constipation worse. The same list includes:
- antacids that contain aluminum or calcium
- calcium channel blockers (a type of blood pressure medicine)
- diuretics (water pills)
- narcotic pain medicines
- anticholinergics and antispasmodics
- seizure medicines, some depression medicines, and Parkinson's medicines
Take one of these plus iron and you're stacking two causes. Don't stop anything on your own. Ask your pharmacist which one is the bigger player.
Some of it may predate the pill, too. In one pregnancy study, 21% of women reported some gut complaint, such as nausea, heartburn, or constipation, when they enrolled, before the study iron began (some were already taking iron on their own). If a prenatal vitamin is making you sick, see our guide to nausea from prenatal vitamins.
And if you take levothyroxine for your thyroid, the label says to take it at least 4 hours apart from iron. If you move your iron to a different meal to ease your gut, keep that gap. Our iron and levothyroxine guide walks through a sample day.
Iron Black Stool vs Warning Signs: When to Call a Doctor
Dark or black stool is common on iron. In one study, black stools became more common as the iron amount went up. Dark color on its own is an expected effect of iron. It doesn't prove the iron is working, either.
Call your doctor right away if constipation comes with any of these signs, which NIDDK lists:
- bleeding from your rectum or blood in your stool
- constant pain in your belly
- not being able to pass gas
- vomiting or fever
- lower back pain
- losing weight without trying
Stool that is black and sticky or tarry, or stool with red or maroon blood, can be a sign of bleeding in the gut, per NIDDK. Seek medical help right away, especially if you feel dizzy or faint or vomit something that looks like coffee grounds. Don't guess whether it's "just the iron." Also see your doctor if self-care hasn't worked, or if colon or rectal cancer runs in your family.
Questions to Ask Your Prescriber
- Is my constipation a reason to change iron forms?
- Would every-other-day dosing work for my iron levels?
- Should I take iron with food or on an empty stomach?
- Which of my other medicines add to the constipation?
- Which stool softener or laxative is safe with my medicines?
How Pillo Helps
If your prescriber moves you to an every-other-day schedule, the hardest part is remembering which day is an iron day. Pillo's Complex Schedule Support lets you set reminders for specific days, so you don't have to count. Its persistent alarm keeps going until you respond, so a planned off day doesn't blur into a forgotten one.
Taking levothyroxine or a calcium antacid too? Ask your pharmacist how far apart to take them, then set each reminder for its own time. Dose Notes let you jot down which iron form you're trying. Your prescriber still sets the schedule. For the rest of your supplements, see our vitamin timing chart.
FAQ
Can iron pills cause constipation or diarrhea?
Both. In a 43-trial analysis of ferrous sulfate, about 12% of people reported constipation and 8% reported diarrhea. The label lists both. Many people get neither.
Does black poop mean iron tablets are working?
No. Dark stool is a common effect of iron, and it got more common at higher iron amounts in one study. It doesn't show whether your iron levels are rising. Only blood tests can show that. Black, sticky or tarry stool, or any red blood, needs a doctor right away.
Why shouldn't you take iron every day?
Every-other-day iron comes up because each pill raises hepcidin, a hormone that lowers absorption from the next dose for about a day, per a 2020 study in women with anemia. Daily iron is still standard for many people. Your prescriber decides based on your blood work.
What helps constipation from iron pills during pregnancy?
Start with fluids, fiber, and movement, and call your OB or midwife before adding any laxative. In a review of trials, mostly in pregnant women, ferrous bisglycinate caused fewer gut side effects than other iron forms. Don't stop prenatal iron without talking to your provider.
When is the best time to take iron tablets to avoid constipation?
We found no trial testing whether time of day changes constipation. The ferrous sulfate label suggests taking it with a meal. Our guide to the best time to take iron covers morning versus night and what to keep apart from iron. Consult your doctor or pharmacist for advice specific to your medications.
