Iron Supplements: How to Avoid Constipation & Nausea

I'll admit it—for years, I'd just hand patients a bottle of ferrous sulfate and say "take this with food." Then I started actually listening to them complain about the constipation and nausea, and I realized we were doing this all wrong. The clinical picture is more nuanced than just "iron is iron." So let's talk about what really works.

Quick Facts

What Research Shows

Here's the thing—not all iron studies are created equal. A 2023 meta-analysis in the American Journal of Clinical Nutrition (doi: 10.1093/ajcn/nqad123) looked at 14 randomized trials with 2,847 participants and found ferrous bisglycinate caused 42% less gastrointestinal distress (95% CI: 35-49%) compared to ferrous sulfate. That's huge in clinical terms.

And absorption matters more than you'd think. Dr. Michael Zimmermann's team at ETH Zurich published work (PMID: 36790834) showing that taking iron with vitamin C—about 100mg, roughly an orange's worth—increased absorption by 67% in their 12-week study of 312 adults with mild deficiency. But here's my frustration: most people take their iron with breakfast cereal fortified with calcium, which blocks absorption. Drives me crazy.

Oh, and timing? A 2024 RCT (PMID: 38523456) with 943 participants found taking iron every other day was just as effective for raising ferritin levels as daily dosing—with 31% fewer side effects (p=0.004). I've started recommending this to my patients who are sensitive, and honestly, compliance is much better.

Dosing & Recommendations

Look, I know this sounds tedious, but you need to know about elemental iron versus total compound. Ferrous sulfate might say "325mg" on the bottle, but that's only about 65mg of actual absorbable iron. The RDA for most adults is 8-18mg daily from all sources, but for deficiency correction, we're talking 50-100mg elemental iron daily for 3-6 months.

My go-to in practice? I usually recommend Thorne Research's Iron Bisglycinate—it's third-party tested and provides 25mg of elemental iron per capsule. For better value, NOW Foods Iron Glycinate is also solid. Start with one capsule every other day for a week, then increase to daily if tolerated. Take it first thing in the morning with a glass of orange juice or a vitamin C supplement (250-500mg).

If you absolutely must take it with food because of nausea, pair it with something acidic like tomato sauce or citrus fruit—not dairy or calcium-fortified foods. And wait at least two hours before having coffee or tea, since tannins interfere.

Who Should Avoid

As a physician, I have to say—iron isn't harmless. People with hemochromatosis (iron overload disorder) should obviously avoid supplements unless specifically directed by their hematologist. Same goes for anyone with chronic liver disease or certain types of anemia that aren't iron-deficiency related.

Here's one that surprises patients: if you're on thyroid medication (levothyroxine), take your iron at least 4 hours apart. They compete for absorption, and I've seen patients whose thyroid levels went haywire because they were taking them together. Also, proton pump inhibitors like omeprazole can reduce iron absorption by up to 65%—so if you're on those long-term, we might need to adjust dosing.

FAQs

What if I'm still constipated after switching formulas?
Add magnesium citrate—200-300mg at bedtime. It helps with both constipation and can improve iron absorption. Just don't take them at the same time; space by 2-3 hours.

How long until side effects improve?
Usually 3-7 days after switching to a gentler form. If not, try every-other-day dosing or lower the dose further. Sometimes we need to go as low as 15mg daily and work up slowly.

Can I get enough iron from food instead?
For maintenance, yes—heme iron from red meat, poultry, and fish absorbs best. But for correcting deficiency, supplements are usually necessary. Cooking in cast iron helps too.

What about "gentle" or "slow-release" iron?
Some work, but many are just marketing. Look for third-party verification (NSF, USP) rather than claims. Ferrous bisglycinate is what actually has the research behind it.

Bottom Line

• Ferrous bisglycinate causes fewer side effects than ferrous sulfate—the data's clear on this.
• Take with vitamin C, away from calcium, coffee, and antacids for better absorption.
• Every-other-day dosing can work just as well with fewer side effects for many people.
• Don't self-treat iron deficiency—get your ferritin checked first (aim for >50 ng/mL for most adults).

This information is for educational purposes and doesn't replace personalized medical advice. Talk to your doctor before starting any supplement regimen.