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Supplements for Constipation: Fiber, Magnesium, Kiwi and What Works First

For most adults with constipation, a fiber supplement, mainly psyllium, is the best first step: in pooled trials more people improved on fiber (grade B), mostly at over 10 g daily. Magnesium oxide also helped in two small trials (grade B), at amounts above the supplement upper limit; anyone with kidney disease should check with a clinician first. See a doctor for blood in the stool, black stools or weight loss.

Published How we grade evidenceEditorial policy

Constipation means hard, infrequent or difficult bowel movements; when it lasts months with no clear cause, it is called chronic idiopathic constipation.

The short version

  • Psyllium fiber helps most often: across 16 trials, 66% of adults responded to fiber supplements against 41% on control, mostly at more than 10 g a day. Grade B (van der Schoot 2022).
  • Magnesium oxide helped in two small placebo-controlled trials, at amounts above the supplement upper limit; avoid it with kidney disease unless your clinician agrees. Grade B (van der Schoot 2023; Chang 2023).
  • Two green kiwifruit a day increased bowel movements in people with constipation; kiwifruit capsules did not. Grade B (Gearry 2023; van der Schoot 2023).
  • Probiotics gave a small, uncertain gain that depends on the strain. Grade C (van der Schoot 2022, probiotics).
  • Iron tablets often upset the gut, and constipation is one of the most common complaints. Grade A (Tolkien 2015).
OptionWhat trials foundSize of effectGrade
Psyllium (fiber)More people improved; more frequent, softer stools66% vs 41% responded (16 fiber trials)B
Magnesium oxideMore people improved68% vs 19% responded (2 trials)B
Green kiwifruit, wholeMore complete bowel movementsAbout 1.5 more a weekB
SennaHelped in one trial; pooled result not significant69% vs 12% in one trialB
PrunesAs good as or better than psylliumSmall trialsC
ProbioticsSmall gain, strain matters57% vs 44% responded (30 trials)C

What works first for chronic constipation?

Fiber, fluids and an over-the-counter laxative. Grade A for polyethylene glycol (PEG), an osmotic laxative medicine that holds water in the stool. The 2023 US gastroenterology guideline strongly recommends PEG, plus bisacodyl or sodium picosulfate (stimulant laxatives) for short-term or rescue use, on moderate-certainty evidence. Fiber, magnesium oxide, senna and lactulose get weaker, conditional recommendations, and prescription options (linaclotide, plecanatide, prucalopride, lubiprostone) come next (Chang 2023). Medicine doses are for your clinician or pharmacist.

Do water and exercise help constipation?

Grade C for both. In a 1998 trial, adults with chronic constipation all ate about 25 g of fiber a day for 2 months; those asked to drink 2 liters of mineral water daily had more bowel movements and used fewer laxatives than those who drank freely (Anti 1998). Pooled, mineral-rich water helped more people than low-mineral water (539 people) (van der Schoot 2024). In 9 exercise trials (680 people), mostly aerobic, symptoms improved, but bias risk was high and results very mixed (Gao 2019).

Does psyllium help constipation?

Grade B. Psyllium is a soluble fiber from a seed husk that turns into a gel with water and softens stool. In 16 trials (1,251 adults), 66% responded to fiber against 41% on control. Psyllium and pectin had clear effects, more people responded only above 10 g a day, and stools came more often mainly after 4 weeks or longer (van der Schoot 2022). Results varied a lot, and fiber caused more wind. Of the fibers the US guideline reviewed, only psyllium appeared to work, on low-certainty evidence (Chang 2023). No trial we found compared times of day; what mattered was the amount, the weeks of use, and a full glass of water with it.

Which fiber is best for constipation?

Grade B for gel-forming soluble fibers such as psyllium; grade C for inulin and flaxseed. A 2026 analysis of 17 trials (1,423 adults) found viscous (gel-forming) soluble fibers eased straining and improved stool frequency and form against placebo; insoluble fibers such as wheat bran ranked best only for overall symptom scores, on very low-certainty evidence (Mou 2026). The US guideline's full text notes that finely ground wheat bran can lower stool water and harden stool (Chang 2023).

  • Inulin, a fiber from chicory root: in 5 trials (252 people), stools became more frequent and softer, but pain and bloating did not ease (Collado Yurrita 2014).
  • Flaxseed: in 77 adults with type 2 diabetes and constipation, 10 g of flaxseed or psyllium baked into cookies twice a day beat placebo cookies over 12 weeks (Soltanian 2019).

Does magnesium help constipation?

Grade B, for magnesium oxide, which draws water into the bowel. Two placebo-controlled trials in Japan used 1.5 g of magnesium oxide a day for 28 days. In 90 adults, overall symptoms improved in 68.3% on magnesium oxide against 11.7% on placebo (Morishita 2021). In 34 women, 70.6% met the main goal against 25.0%, and stools softened, though complete bowel movements did not rise significantly (Mori 2019). Pooled, 68% responded against 19% (van der Schoot 2023). The US guideline rated the certainty very low; the British dietitians' guideline rated it moderate (Chang 2023; Dimidi 2025).

That amount is far above the limit. Upper limit for adults: 350 mg of magnesium a day from supplements, because more can cause diarrhea; take more only with blood tests or your clinician's advice (NIH ODS; check your stack). Magnesium builds up in the blood when the kidneys don't clear it, so people with kidney disease must ask their clinician before taking any. The US guideline says to avoid magnesium oxide with kidney insufficiency (Chang 2023). See our magnesium page.

Which magnesium is best for constipation?

Magnesium oxide, the form with placebo-controlled trials: both trials in the 2023 review used it (grade B, van der Schoot 2023). We found no randomized trial of magnesium citrate or magnesium glycinate for chronic constipation, so neither has a tested amount. We also found no study of whether glycinate causes constipation; magnesium supplements more often loosen stools, which is why the upper limit is set for diarrhea.

Do kiwifruit or prunes help constipation?

Grade B for kiwifruit, C for prunes. In a crossover trial, people with chronic constipation (60), constipation-type irritable bowel syndrome (61) and no gut problems (63) ate 2 green kiwifruit or took 7.5 g of psyllium daily for 4 weeks each. On kiwifruit, both constipated groups gained about 1.5 complete bowel movements a week and had less gut discomfort, with no significant side effects (Gearry 2023). In 79 US adults, 2 kiwifruit, 100 g of prunes or 12 g of psyllium a day all raised complete bowel movements over 4 weeks, and kiwifruit caused the fewest side effects (Chey 2021).

In a crossover trial of 40 adults, 50 g of prunes twice a day beat psyllium for complete bowel movements and stool softness over 3 weeks (Attaluri 2011). Pooled, prunes did no better than psyllium on stool frequency, while kiwifruit did slightly better (van der Schoot 2024); a 2022 review rated the kiwifruit evidence low certainty (Eltorki 2022).

Do probiotics help constipation?

Grade C. Across 30 trials, 57% responded to probiotics against 44% on control. Stools came more often but were no softer; Bifidobacterium lactis raised frequency, mixtures and two other strains did not, and the authors warned of high risk of bias and very mixed results (van der Schoot 2022, probiotics). The 2025 British dietitians' guideline rated this evidence low to very low and said it is unclear which strains work (Dimidi 2025).

Does senna work for constipation?

Grade B, and the reviews disagree. Senna is a plant stimulant laxative, sold as a medicine in many countries. In the 90-adult trial above, 69.2% improved on senna against 11.7% on placebo over 28 days, with no severe side effects (Morishita 2021). Pooled with a second trial, the gain fell short of significance (van der Schoot 2023). The US guideline rates the evidence low certainty, with a conditional recommendation (Chang 2023), while a 2021 review of over-the-counter products rated it good (Rao 2021). Our database grades senna C from two small older trials and doesn't yet count the 2021 trial.

Does iron cause constipation?

Grade A that iron tablets upset the gut. In 20 placebo-controlled trials (3,168 adults), ferrous sulfate, the usual iron tablet, raised the odds of gut side effects (odds ratio 2.32), with no clear link to dose (Tolkien 2015). The review's full text adds that in the 27 trials that counted it, about 12% of people taking ferrous sulfate reported constipation. In 150 young women with low iron stores, iron every other day caused fewer gut side effects than daily iron at the same total amount (grade C, von Siebenthal 2023). Ask your clinician about the schedule; see our iron page.

Calcium: grade B for more gut complaints in general, C for constipation itself. In 7 trials, gut complaints such as constipation, cramping and bloating were reported by 14.1% on calcium against 10.0% on placebo (Lewis 2012); in a small crossover trial of 27 healthy women, 500 mg a day of calcium did not change stool frequency or form (Alyousif 2016).

What we'd skip

  • Kiwifruit capsules and powders. Three small trials found no change in stool frequency or form; the whole fruit is what helped (van der Schoot 2023).
  • Aloe as a laxative. In 2002 the FDA ruled that aloe and cascara sagrada laxative ingredients are not generally recognized as safe and effective in over-the-counter products (Federal Register; FDA 2002). In a case report, a 73-year-old woman developed hepatitis (liver inflammation) while taking aloe capsules for constipation, which cleared when she stopped (Bottenberg 2007). The one aloe trial we found mixed it with psyllium and celandine, so it can't show what aloe alone does (Odes 1991).
  • Senna teas sold for slimming or "cleansing", taken daily for long periods. In a case report, a woman who drank a liter a day of tea made from 70 g of senna fruit for over 3 years developed liver failure and kidney damage (Vanderperren 2005).
  • Synbiotics (probiotic plus fiber mixes). In 4 trials, they had no effect on stool output or symptom scores (van der Schoot 2022, probiotics).

Safety and interactions

  • Magnesium overload (hypermagnesemia): in a case series, 4 people over 65 with kidney problems developed severe magnesium overload from magnesium oxide taken for constipation, and one died (Yamaguchi 2019). Among older hospital patients prescribed magnesium oxide, about half of those with both poor kidney clearance and 900 mg or more a day developed it (Ishii 2023). One fatal case occurred with normal kidney function in a woman taking magnesium laxatives regularly (Bokhari 2018).
  • Interactions: magnesium, iron and calcium can cut how much levothyroxine you absorb (can I take iron, calcium or magnesium with levothyroxine?); for water pills and other blood pressure drugs, see can I take magnesium with blood pressure medicine?. Ask your prescriber or pharmacist.
  • Psyllium and other bulk fibers: an FDA labeling rule warns they can block the throat if taken without enough fluid or by people who have trouble swallowing (21 CFR 201.319). Build up slowly; extra wind is common (van der Schoot 2022).
  • Senna and other anthraquinone laxatives: pooled observational studies found no significant link with bowel cancer, but a trend that needs better studies (Lombardi 2022). Ask a clinician before daily long-term use.
  • Several supplements at once: check your stack. No interaction studies were found for kiwifruit or prunes with common medicines.

When to see a doctor

  • Blood in or on the stool, or black, tarry stools.
  • Unexplained weight loss, or iron-deficiency anemia (low iron on a blood test).
  • Age over 50 with new constipation or a change in bowel habit.
  • Severe belly pain, vomiting, or a swollen belly with no stool or wind: get urgent care; the bowel may be blocked.
  • Constipation that began after starting a medicine, for example opioid pain relief or iron. Don't stop a prescribed medicine on your own; ask the prescriber or pharmacist.
  • No improvement after about 4 weeks of fiber and fluids.
  • Children: ask a clinician. Pregnant or breastfeeding: ask your doctor or midwife first.

FAQ

What is the best supplement for constipation? Psyllium fiber has the most support (grade B): across 16 trials, 66% of adults responded to fiber supplements against 41% on control. Magnesium oxide also helped in two small trials (grade B), at amounts above the supplement upper limit.

How much magnesium citrate should I take for constipation? We found no randomized trial of magnesium citrate for chronic constipation, so there is no tested amount. The trials used 1.5 g of magnesium oxide a day, above the 350 mg adult upper limit for supplements; ask a clinician first, and avoid it with kidney disease.

Does magnesium glycinate cause constipation? We found no study of whether it does. Magnesium supplements more often loosen stools, which is why the adult upper limit is set for diarrhea.

When is the best time to take psyllium husk? No trial we found compared times of day. What mattered in the trials was more than 10 g a day, 4 weeks or longer, and a full glass of water with it.

Does iron cause constipation? Often. In 20 placebo-controlled trials, ferrous sulfate raised the odds of gut side effects (odds ratio 2.32, grade A), and constipation was among the most common.

Sources

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