Can Iron Supplements Cause Diarrhea?

Side effects of iron pills include unpleasant gastrointestinal issues, such as diarrhea, constipation and nausea. There are "gentle" and slow-release iron supplements available that can ease these effects on bowel movements, and you can also try increasing your iron levels through your diet.

One of the side effects of iron supplements is diarrhea. (Image: eyenigelen/E+/GettyImages)

Tip

The effects of iron supplements on bowel movements include diarrhea, constipation and darkened stools.

What Are Iron Supplements?

Iron plays a critical role in producing blood. UCSF Health explains that around 70 percent of the iron in your body is stored in red blood cells (hemoglobin) or muscle cells (myoglobin), where it helps transport, store and release oxygen.

Roughly 25 percent of your iron is stored in cells circulating in your blood. UCSF estimates that the average adult male has 1,000 milligrams of stored iron, which is enough for roughly three years. In comparison, the average adult woman has about 300 milligrams of stored iron circulating in her blood, which would last about six months.

To prevent your iron stores from depleting, keep in mind the Recommended Dietary Allowances (RDA) provided by Oregon State University's Linus Pauling Institute:

  • 8 milligrams a day for men aged 19-50
  • 18 milligrams a day for women aged 19-50
  • 8 milligrams a day for men and women over 51
  • 27 milligrams a day for pregnant adults
  • 9-10 milligrams a day for breastfeeding adults

The Institute also states that iron deficiency is "the most common nutrient deficiency in the U.S. and the world." Fortunately, eating an iron-rich diet can help, and there are different types of iron supplements you can take.

According to the National Institutes of Health Office of Dietary Supplements, ferrous and ferric iron salts including ferrous sulfate, ferrous gluconate, ferric sulfate and ferric citrate are common forms of iron found in oral supplements. The NIH estimates that 14 to 18 percent of Americans take an iron supplement, including 60 percent of breastfeeding adults and 72 percent of pregnant people.

Do Iron Pills Cause Diarrhea?

According to Dartmouth College Health Services, the effects of iron supplements on bowel movements include both constipation and diarrhea. Another side effect of iron pills is nausea.

In a literature review published in the journal PLOS ONE in 2015, researchers looked at the side effects of iron pills in 43 different trials on 6,831 adults. A meta-analysis of the data confirmed that taking ferrous sulfate supplements "is associated with a significant increase in gastrointestinal-specific side-effects" (20 trials compared taking oral ferrous sulfate supplements with taking a placebo, and 23 trials compared taking oral ferrous sulfate supplements to taking intravenous iron).

In the 27 studies reporting constipation as a side effect, 12 percent of people taking iron pills experienced constipation. Participants in 30 studies reported nausea as a side effect, with roughly 11 percent affected. Finally, iron pills caused diarrhea in 25 different studies, on 8 percent of participants.

The results showed that the dosage of the iron supplements did not affect how likely people were to experience side effects from taking them.

Iron Supplement Side Effects

MedLinePlus explains that iron supplements are absorbed best if you take them on an empty stomach. However, if iron pills cause diarrhea, constipation or nausea, taking the supplements with a small amount of food might help. If you're taking liquid iron supplements, they may stain your teeth, so take the supplement through a straw to avoid this.

Some foods inhibit iron absorption, including eggs, coffee and milk. MedlinePlus says to avoid drinking milk or caffeinated beverages, taking antacids or eating high-fiber foods like whole grains and bran for about two hours before taking your iron supplement.

Another potential side effect of iron pills involves interactions with other medications you take.

  • According to Harvard Health Publishing, iron supplements can "diminish the effects" of levothyroxine taken for an underactive thyroid.
  • The University of Rochester Medical Center says that medicines in the tetracycline family will bind with iron, decreasing the absorption of both. Tetracycline is used to treat various bacterial infections and acne.
  • Iron can affect levodopa, a drug used to treat Parkinson's disease.
  • Iron supplements can also affect the amount of methyldopa in your body, a drug used to treat high blood pressure.
  • The Cleveland Clinic warns that iron supplements can interfere with alendronate, which is prescribed for osteoporosis and Paget's disease.
  • Quinolone antibiotics, used to treat respiratory and urinary tract infections, may also be affected by iron supplements.

If you're taking any prescription medications, check with your doctor before you begin taking over-the-counter iron supplements.

When Iron Supplements Cause Diarrhea

If your doctor has recommended iron supplements but your iron pills cause diarrhea, ask about taking "gentle" iron supplements or slow-release options. You may also be able to increase your iron levels by tweaking your dietary intake. Good sources of iron include:

  • Chicken liver. Three ounces of cooked chicken livers provides 142 calories and almost 10 milligrams of iron. Chicken livers are also inexpensive and a good source of protein.
  • Oysters. A 3-ounce serving of raw Pacific oysters provides 69 calories and over 5 milligrams of iron. Just note that eating raw oysters can be dangerous for young children, senior citizens, pregnant people and anyone suffering from a compromised immune system.
  • Fortified breakfast cereal. One cup of fortified puffed-wheat cereal contains 44 calories and almost 4 milligrams of iron. Some fortified cereals are high in added sugar, so look for options with plenty of fiber and fewer refined carbs.
  • Clams. A 3-ounce serving of cooked clams provides 126 calories and almost 3 milligrams of iron — plus almost 22 grams of protein.
  • Beef. A 3-ounce serving of 70-percent-lean meat, pan-browned, provides 230 calories and over 2 milligrams of iron.
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