What causes constipation?

Most often too little fiber or fluid, travel or a change in routine, too little activity, or a supplement or medicine such as iron, calcium, antacids or opioid pain medicines. Medical causes include IBS, an underactive thyroid, diabetes and celiac disease.

Can iron supplements cause constipation?

Yes. In 43 trials, ferrous sulfate more than doubled the odds of gut side effects compared with a placebo. Ferrous bisglycinate caused fewer gut side effects in pregnancy trials. If iron was prescribed for anemia, ask the prescriber about the form or schedule rather than stopping it.

Which calcium is less constipating, carbonate or citrate?

Both can cause gas, bloating and constipation. There is little head-to-head evidence that one is gentler. MedlinePlus says calcium carbonate should be taken with meals. Calcium citrate absorbs with or without food. MedlinePlus lists spreading calcium through the day, more fluids and fiber, and then another form if those don't help; ask a pharmacist or clinician before changing.

Does drinking more water help constipation?

Only if you are dehydrated, according to the American Family Physician review. Fluids do help fiber work, so drink more when you add fiber.

What is the best supplement for constipation?

Psyllium fiber and magnesium oxide have the best evidence among supplements (Moderate on our scale). The 2025 British Dietetic Association guideline recommends both; it rated magnesium oxide's evidence moderate and psyllium's low, because the trials were small. Probiotics may help, but it is unclear which strains (Limited). Magnesium needs a clinician's OK with kidney disease.

Can thyroid problems cause constipation?

Yes. Hard stools or constipation is an early sign of an underactive thyroid. A TSH blood test is usually checked first.

When is constipation serious?

See a doctor right away for blood in the stool, weight loss without trying, constant belly pain, vomiting, fever or being unable to pass gas. Black, tarry stools or a lot of blood means call 911. A new change in bowel habits, especially over age 50, is worth a visit.

Constipation: the everyday causes, the supplements

Q: What counts as constipation?

NIDDK defines it as fewer than three bowel movements a week, or stools that are hard, dry or painful to pass. About 16 in 100 adults have symptoms, and 33 in 100 adults aged 60 and older.

The distinctions

01
Check what you take before you add something
Iron, calcium, antacids and opioid pain medicines are among the commonest causes of constipation. Looking at what you already take, with the prescriber for anything prescribed, often finds the cause before any test or new supplement does.
02
Fiber has the evidence; water alone does not
Psyllium fiber has Moderate evidence (several small trials pointing the same way) for more frequent, softer stools. Drinking extra water on its own helps only if you are dehydrated, though fluids help fiber work.

Constipation usually has an everyday cause. Too little fiber, not enough to drink, travel, sitting most of the day, or a new supplement or medicine are the common ones. ² Iron and calcium supplements, antacids and opioid pain medicines are easy to miss. ² For most people, more fiber, a regular toilet routine and a psyllium fiber supplement do the most. ⁴⁸ A few medical causes, such as an underactive thyroid or high calcium, show up on simple blood tests. ⁵¹³

When to get help now

Most constipation is not dangerous. These signs are the exceptions.

Call 911 or your local emergency number

Black, tarry stools, or a lot of blood in your stool ¹⁷
Vomiting blood or material that looks like coffee grounds ¹⁷
A swollen, painful belly with vomiting, and you can't pass stool or gas ²¹⁶

See a doctor soon, the same day if you can

Small amounts of blood in your stool or on the paper ²¹⁷
Losing weight without trying ²
Belly pain that won't go away, fever or lower back pain with the constipation ²
A new change in bowel habits or narrower stools, especially over age 50 ⁶¹⁵
A family history of colon or rectal cancer ²
Constipation that doesn't improve with fiber, fluids and a routine ²

LifeFrom can't examine you or assess a symptom. If something feels seriously wrong, trust that and get care rather than waiting on a test.

What counts as constipation?

NIDDK defines constipation as “fewer than three bowel movements a week,” or stools that are hard, dry, lumpy or painful to pass. ¹ It is more common in women, during pregnancy and in older adults. ¹

16 in 100adults have symptoms of constipationRising to 33 in 100 adults aged 60 and older. Source: NIDDK. ¹

What can cause it, most likely first

Ranked from most to least likely in adults in general. The order shifts with age, sex and history: medicines and slower movement matter more after 60, and pregnancy is a common cause for women. ¹²

01
Too little fiber or fluidEverydaycommon
Adults need 22 to 34 grams of fiber a day, depending on age and sex. ³ Most US adults eat about 16 grams. ⁶ Fiber adds bulk to stool and makes bowel movements more frequent. ⁶ Fluids help fiber work, but more water alone helps only if you are dehydrated. ³⁶
02
Travel, routine changes and sitting stillEverydaycommon
NIDDK lists travel, changes in diet or routine, ignoring the urge to go and too little physical activity among the causes. ² Holding it in on a busy workday is a common one. Studies on exercise are mixed, though small studies link a mostly seated day with more constipation. ⁶
03
Iron or calcium supplementsEverydaycommon
Iron supplements are on the NIDDK list. ² In 43 trials, ferrous sulfate, the most common iron form, more than doubled the odds of gut side effects compared with a placebo. ⁹ Calcium can cause gas, bloating and constipation too. ¹¹ If you take iron for anemia, ask the prescriber about the form or schedule before changing anything. See the comparison below for calcium.
04
MedicinesEverydaycommon
Opioid pain medicines are a common cause. So are antacids with aluminum or calcium. Some medicines for depression, blood pressure, fluid (water pills), seizures, Parkinson's disease and gut spasms can cause it too. ² NIDDK says: “Don't change or stop any medicine or supplement without talking with a health care professional.” ⁴
05
Irritable bowel syndrome (IBS)Medicalcommon
In IBS with constipation, belly pain is tied to bowel movements. Doctors think of it when symptoms come at least once a week for 3 months, and first started at least 6 months ago. ²⁰ No blood test diagnoses IBS; tests rule out look-alikes such as celiac disease. ²⁰ See bloating.
Can be linked with Irritable bowel syndrome
06
Underactive thyroidMedicalless common
“Hard stools or constipation” is one of the early signs of hypothyroidism (an underactive thyroid), along with feeling cold and tired. ¹² See how to read your thyroid panel.
Can be linked with Hypothyroidism
TSH
A high TSH most often means an underactive thyroid, and TSH is usually checked first. ¹⁸
Free T4
A low free T4 with a high TSH fits an underactive thyroid. ¹²¹⁸
07
DiabetesMedicalless common
Diabetes is on the NIDDK list of conditions that can cause constipation. ² See blood sugar explained.
Can be linked with Type 2 diabetes
HbA1c
HbA1c reads average blood sugar over about 3 months: below 5.7% is normal, 5.7% to 6.4% is prediabetes and 6.5% or above is diabetes. ¹⁹
08
Celiac diseaseMedicalless common
Celiac disease, an immune reaction to gluten, is on the NIDDK list of causes. ² The blood test only works while gluten is still in your diet: MedlinePlus says “you'll need to continue to eat foods with gluten for a few weeks before testing.” ¹⁴
Can be linked with Celiac disease
tTG-IgA
A positive or high tTG-IgA (tissue transglutaminase antibody) points to celiac disease and is usually followed by a biopsy; it is the most commonly used celiac test. ¹⁴
Total IgA
A low total IgA can make tTG-IgA read falsely negative, which is why it is often checked alongside. ¹⁴
09
High blood calciumMedicalrare
In primary hyperparathyroidism, four small glands in the neck make too much hormone, and blood calcium rises. Most people have no symptoms. Worse cases bring constipation, thirst, nausea and confusion. ¹³ It is most common between 50 and 60, and in women. ¹³
Can be linked with Hyperparathyroidism
Calcium
A high blood calcium is often found on a routine panel; with a high or upper-normal PTH it points to primary hyperparathyroidism. ¹³
PTH
A high PTH, or one in the upper part of normal when calcium is high, fits primary hyperparathyroidism. ¹³
10
Colon cancer or a blockageMedicalrare
A new change in bowel habits, narrower stools, bleeding or weight loss can be signs of colon cancer. Clinicians send people with these signs for a colonoscopy. ⁶ Without these warning signs, routine blood tests and colonoscopy are not recommended for constipation. ⁶ Routine colorectal cancer screening starts at 45 for people at average risk. ¹⁵
FIT (stool blood test)
A positive result means blood was found in the stool and leads to a colonoscopy; it is a screening test, and symptoms still need a clinician. ⁵¹⁵
Hemoglobin
A low hemoglobin (anemia) with constipation is a reason for a clinician to look further; a blood count checks it. ⁵

These are things a symptom like this can be linked with, not a diagnosis. Several can sit together, and a clinician who knows your history puts them in order for you.

Supplements and medicines that can cause constipation

What you take	How likely	What to know
Iron (ferrous sulfate)	Common	Gut side effects about twice as likely as placebo ⁹
Iron (ferrous bisglycinate)	Less common	Fewer gut side effects than iron salts in pregnancy trials; evidence Limited ¹⁰
Calcium carbonate or calcium citrate	Common	Both can cause gas, bloating and constipation ¹¹
Antacids with aluminum or calcium	Common	Listed by NIDDK ²
Opioid pain medicines	Common	Listed by NIDDK; tell the prescriber ²
Some depression, blood pressure, seizure, Parkinson's and bladder medicines	Varies	Listed by NIDDK; ask before changing ²⁴

Calcium carbonate or calcium citrate?

Calcium carbonateBest taken with meals

Cheaper; also in many antacids

With foodMedlinePlus says to take it with meals

Low stomach acid or acid reducersAbsorbed less well

Gas, bloating, constipationCan happen

Evidence it constipates moreLimited

Calcium citrateAbsorbed with or without food

Costs more

With foodWorks on an empty or full stomach

Low stomach acid or acid reducersAbsorbed better

Gas, bloating, constipationCan happen

Evidence it constipates lessLimited

From MedlinePlus and a clinical review of calcium forms. ¹¹²¹ Head-to-head trials of constipation are few, so neither form is proven gentler. For calcium side effects, MedlinePlus lists spreading calcium through the day, more fluids and fiber, and then another form of calcium if those don't help; a pharmacist or clinician can help with that choice. ¹¹

What helps constipation, ranked by evidence

Two recent guidelines graded the options. The 2025 British Dietetic Association guideline reviewed 75 trials of foods and supplements. ⁸ The 2023 American Gastroenterological Association and American College of Gastroenterology guideline reviewed medicines. ⁷ The table puts them on one scale: Moderate means several placebo-controlled trials pointing the same way. Even over-the-counter options are worth checking with a pharmacist or clinician first, especially if you have kidney disease or take other medicines.

Option	Evidence	What the studies found
Psyllium fiber supplement	Moderate	Three placebo-controlled trials all found more people improved, with more frequent, softer stools; trials were small, so the guideline rated certainty low but still recommended it ⁸
Magnesium oxide	Moderate	Two small trials (94 people) found more people improved, with softer, more frequent stools; needs a clinician's OK with kidney disease ⁶⁸
Polyethylene glycol (an over-the-counter laxative)	Strong	Many trials; the US guideline's strongest recommendation ⁷
Kiwifruit	Limited	Whole kiwifruit did about as well as psyllium ⁸
Prunes	Limited	About as good as psyllium for softness and straining ⁸
Mineral-rich water	Limited	Helped more people than low-mineral water ⁸
Probiotics	Limited	May help, but unclear which strains ⁶⁸
Exercise	Limited	No consistent evidence it relieves constipation ⁶
More water on its own	Not supported	Helps only if you are dehydrated ⁶
Inulin and mixed fiber blends	Not supported	No clear benefit, and more gas ⁸

1Add fiber slowlyAdd fiber a little at a time so your gut gets used to it, with whole grains, beans, fruit and vegetables. ³
2Drink with itDrink plenty of liquids when you add fiber or a fiber supplement. ⁴
3Pick a timeTry for a bowel movement at the same time each day, 15 to 45 minutes after breakfast. ⁴
4Go when you feel itUse the bathroom as soon as you feel the urge. ⁴
5Raise your feetRelax, and put your feet on a footstool. ⁴

Do I need tests for constipation?

Often not. The American Family Physician review says routine blood tests, imaging or colonoscopy are not recommended without warning signs. ⁶ When a clinician does test, “Blood tests can show signs of anemia, hypothyroidism, and celiac disease,” and stool tests look for blood. ⁵ Routine blood panels often pick up a high calcium. ¹³

The panel that checks these

If a clinician tests, TSH, calcium, HbA1c and a blood count usually come first, because they check the commonest medical causes on one draw. ⁵¹³ A celiac test needs gluten in the diet for a few weeks first. ¹⁴ None of these needs fasting, apart from a fasting glucose if one is added.

TSHUnderactive thyroid
Free T4Underactive thyroid
HbA1cDiabetes
tTG-IgACeliac disease
Total IgACeliac disease
CalciumHigh blood calcium
PTHHigh blood calcium
FIT (stool blood test)Colon cancer or a blockage
HemoglobinColon cancer or a blockage

Already have results?Add them to LifeFrom and it reads these markers side by side, against each other and over time, in plain language. Free, on the web or in the app.

Add your results Download the app

LifeFrom doesn't sell blood tests. It reads results from any lab and helps you prepare for your own doctor; it doesn't diagnose.

Getting a useful appointment

Keep a two-week note of bowel movements, what they were like, fiber, fluids and activity.
List every supplement and medicine with its start date, especially iron, calcium, antacids and pain medicines. ²
Mention blood, weight loss, belly pain and any family history of colon cancer or celiac disease. ²⁶
Say whether you have had colorectal cancer screening if you are 45 or older. ¹⁵
One result is a snapshot; see why trends beat single readings.

What may help

Most people improve with the everyday steps, and a fiber supplement has the best evidence among supplements. None replaces checking the red flags above.

Start with the everyday things

Work up toward 22 to 34 grams of fiber a day from food, adding it slowly. ³
Psyllium has the best evidence of any fiber supplement (Moderate); the trials ran for about 4 weeks or more. ⁸
Kiwifruit and prunes did about as well as psyllium in trials (Limited). ⁸
Keep a regular toilet time after breakfast, and go when you feel the urge. ⁴
Drink enough to stay hydrated, especially with extra fiber. ⁴⁶

Supplements people look at

Iron is left off on purpose: low iron needs its cause found, and a clinician decides on iron after a test. Supplements support general wellness. They don't treat the causes above, and if a test finds one, treating that comes first. If you take medicines, are pregnant or have a health condition, check with your clinician before starting anything.

Questions to ask your clinician

Could any of my supplements or medicines be causing this, and is there another form?
Do my symptoms fit IBS, or should we rule out celiac disease or a thyroid problem first?
Should my calcium, TSH or blood sugar be checked?
Am I due for colorectal cancer screening?

This page is general education, not medical advice or a diagnosis. The clinician who knows your history and can examine you is the one to read your results. If bloating or tiredness comes with the constipation, see bloating and fatigue.

Free in LifeFromAdd the lab results you already have, and LifeFrom reads each marker next to the others it moves with, in plain language, with the sources it used. Free, on the web or in the app.

Start on the web Download the app

Citations

Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.

Constipation: the everyday causes, the supplements and medicines behind it, and the tests that check the rest