Does flaxseed help with constipation?

Probably a little, and the trials behind that are small. In an open randomized trial of 90 adults in China with functional constipation, 1.8 oz (50 g) of flaxseed flour a day for 4 weeks raised the median number of bowel movements from 2 to 7 a week, against 2 to 6 on lactulose, a standard laxative. There was no placebo group and no blinding was reported. No meta-analysis has pooled flaxseed for constipation, so every number on this page comes from a single study.

Unsettled. Two small trials point the same way and two point nowhere. Flaxseed flour did somewhat better than lactulose in one open trial, by the authors' account, and beat placebo cookies in one single-blind trial in people with diabetes. In people with irritable bowel syndrome and in heart patients in intensive care it made no measurable difference. We found no blinded placebo trial of flaxseed alone in ordinary adults with constipation, which is the trial that would settle it.

What the trials found

Against a laxative

The 90-person trial compared 1.8 oz (50 g) of flaxseed flour a day with 1 tbsp (15 mL) of lactulose for 4 weeks. Both groups went from a median of 2 bowel movements a week to 7 and 6. The Wexner constipation score fell from 14 to 6.5 with flaxseed and from 15 to 9 with lactulose. The authors concluded flaxseed was somewhat more effective and improved quality of life more, though the abstract gives no between-group statistics, and the full text is not in PubMed Central.

Against placebo, in people with diabetes

In Iran, 53 people with type 2 diabetes and chronic constipation ate cookies with 10 g of flaxseed twice a day, or placebo cookies, for 12 weeks. At the end, 11 people on flaxseed (42.3 percent) rated their symptoms at least somewhat better, against 2 on placebo (7.4 percent). The trial was single-blind. The same researchers published a three-arm version with 77 patients and a psyllium arm, which found flaxseed improved constipation symptoms (P = 0.002) and stool consistency (P below 0.001) against placebo. The two reports share authors, design and dose and are probably the same patients, so read them as one trial.

Where it made no difference

In an open pilot trial of 40 adults with irritable bowel syndrome, two tablespoons a day of whole or ground linseed for 4 weeks did not change stool frequency or consistency in any group. That trial had 31 completers and was too small to rule out an effect. The second null trial recruited 70 heart attack patients in intensive care who were not constipated. Given for 4 days, 18 g of flaxseed powder a day gave a mean of 1.86 bowel movements against 1.6 on magnesium hydroxide, a difference that did not reach significance (P = 0.234), and the first stool came sooner on magnesium hydroxide.

Smaller and weaker evidence

In a double-blind crossover trial of 43 older adults with mild constipation, mean age 76, a yogurt containing galacto-oligosaccharides, prunes and linseed raised bowel movements to 8.0 a week against 7.1 on plain yogurt over 3-week periods. Linseed was one of three active ingredients and the trial was industry funded, so its own share is unknown. In healthy young adults who were not constipated, 1.8 oz (50 g) of flaxseed a day for 4 weeks raised bowel movements by 30 percent a week. An uncontrolled study of 60 older adults in China with functional constipation reported a rise from 3.8 to 6.2 bowel movements a week on 1.8 oz (50 g) of flaxseed powder for a month. With no control group, part of that rise may have happened anyway.

Flaxseed oil is a different product, without the seed's fiber and mucilage. In one trial in people with chronic kidney disease, flaxseed oil, olive oil and mineral oil all reduced constipation symptom scores from baseline, so the oil did no better than its comparators.

Who should be careful

Not for everyone. According to the US National Center for Complementary and Integrative Health, higher doses of flaxseed or flaxseed oil can cause bloating, fullness and diarrhea, and it gives no dose at which this starts. The same agency says some studies suggest flaxseed is possibly unsafe in pregnancy, though the evidence is not conclusive. If you are pregnant, that is a reason to ask before using it as a remedy.

NCCIH also says there are theoretical reasons flaxseed might interact with blood thinners such as anticoagulant and antiplatelet drugs, and no clinical cases are cited for this. In a review of 1,491 documented supplement and drug pairs, flaxseed was among the three supplements with the most documented contraindications. That is a count of written warnings, not of harm, and it covers supplement doses rather than a spoonful of seed. If you take a blood thinner or several regular medicines, check with a pharmacist.

Allergy is rare and can be serious. A case report describes anaphylaxis with linseed-specific IgE in a 39-year-old woman after eating linseed, and a review of seed allergies found the flaxseed literature is case reports rather than series, so nobody can say how common it is.

The trial doses of 50 g a day were taken as flour or powder. In healthy women eating 1.8 oz (50 g) of raw ground flaxseed a day for 4 weeks, urinary thiocyanate, a marker of cyanide exposure, rose 2.2-fold. That was an uncontrolled 1993 study. The cyanide-forming compounds were not detected in baked muffins made with 5.3 oz (150 g) of flaxseed per kg. EU law limits hydrocyanic acid in linseed for this reason, noting that ground raw linseed gives more exposure than whole or heat-treated seed. In 34 healthy adults, up to 40 g a day of milled flaxseed baked into a muffin for 30 days caused no reported adverse events, with 8 to 9 people per dose. In the 12-week diabetes trial, 20 g a day in cookies caused no serious adverse events, and the 60-person uncontrolled study reported no adverse reactions at 50 g.

What expert bodies say

A 2021 systematic review in the American Journal of Gastroenterology graded 41 randomized trials of over-the-counter constipation remedies. Only polyethylene glycol and senna got good evidence, grade A. A yogurt with galacto-oligosaccharides, prunes and linseed oil got moderate evidence, grade B, alongside psyllium, prunes and kiwi. Flaxseed on its own was not graded, and the review pooled no numbers.

The NCCIH fact sheet we used speaks to flaxseed safety, and those cautions are in the section above.

More on the seed itself is on the flaxseed page.

How we searched

What would change this answer

• A blinded, placebo-controlled randomized trial of ground flaxseed in adults with constipation diagnosed by the Rome IV criteria. None was registered on ClinicalTrials.gov on 6 October 2026.
• A meta-analysis that pools the existing trials with a proper assessment of bias, or the two unindexed reviews becoming traceable.
• A trial that tests the dose. The positive trials used 20 g and 50 g a day, and no study compared doses for constipation or reported how often bloating limits them.
• Full texts of the lactulose trial, so that the comparison between groups can be read directly.

Sources

1. ev-flco-01 · Randomized controlled trial(s) · randomized controlled trial, open, active comparator (lactulose 15 mL/day) · n = 90
   The median defecation frequency per week increased significantly (2 to 7 for flaxseed and 2 to 6 for lactulose, p<0.001 for both groups).
   
   Who: adults in China with functional constipation diagnosed by the Rome IV criteria
   Effect: median defecation frequency 2 to 7 per week with flaxseed vs 2 to 6 with lactulose; Wexner score 14 to 6.5 vs 15 to 9
   Certainty: no placebo arm, no blinding reported, within-group changes; single center country
   Asia Pac J Clin Nutr, 2020 · checked 2026-10-06 · we read the abstract
2. ev-flco-02 · Randomized controlled trial(s) · randomized controlled trial, open, active comparator (lactulose 15 mL/day) · n = 90
   Flaxseed flour is somewhat more effective at increasing defecation frequency than lactulose, improving bowel movements and promoting life quality of subjects with chronic functional constipation in the Chinese population.
   
   Who: adults in China with functional constipation diagnosed by the Rome IV criteria
   Effect: PAC-QOL score change -1.34 with flaxseed vs -0.66 with lactulose
   Certainty: between-group statistics not given in the abstract; full text not in PMC
   Asia Pac J Clin Nutr, 2020 · checked 2026-10-06 · we read the abstract
3. ev-flco-03 · Randomized controlled trial(s) · single-blind randomized placebo-controlled trial, 12 weeks · n = 53
   On the global constipation symptom survey, 2 (7.4%) patients who received placebo cookies and 11 (42.3%) patients who received flaxseed reported improvement of symptoms and rated their improvement as at least somewhat better (+ 2) when compared to baseline symptoms.
   
   Who: patients with type 2 diabetes and chronic constipation in Iran
   Effect: improvement at least somewhat better: 11 (42.3%) flaxseed vs 2 (7.4%) placebo; global symptom score change 2.46 vs 0.41, P = 0.008
   Certainty: small, single-blind; same group published a 77-patient version with a psyllium arm (30661699), likely overlapping
   Nutrition & Metabolism, 2018, 15:36 (Soltanian, Janghorbani) · checked 2026-10-06 · we read the fulltext
4. ev-flco-04 · Randomized controlled trial(s) · single-blind randomized placebo-controlled trial, 12 weeks · n = 53
   Flaxseed cookies were well tolerated, with no serious adverse events.
   
   Who: patients with type 2 diabetes and chronic constipation in Iran
   Effect: no serious adverse events
   Certainty: minor GI effects not tabulated
   Nutrition & Metabolism, 2018, 15:36 (Soltanian, Janghorbani) · checked 2026-10-06 · we read the fulltext
5. ev-flco-05 · Randomized controlled trial(s) · single-blind randomized placebo-controlled trial, three arms (flaxseed, psyllium, placebo), 12 weeks · n = 77
   The change from baseline of constipation symptoms (P = 0.002), stool consistency (P < 0.001), weight (P < 0.001), BMI (P < 0.001), FPG (P = 0.004), cholesterol (P = 0.010), LDLC (P = 0.031), and cholesterol/HDLC ratio (P = 0.019), was significantly improved in both flaxseed and psyllium groups than in the placebo group.
   
   Who: constipated patients with type 2 diabetes, Rome III criteria
   Effect: change in constipation symptoms P = 0.002 and stool consistency P < 0.001 vs placebo; no effect size in abstract
   Certainty: likely overlaps with the 53-patient report (29760761); superiority over psyllium not formally tested in abstract
   Clinical Nutrition ESPEN, 2019, 29:41-48 (Soltanian, Janghorbani) · checked 2026-10-06 · we read the abstract
6. ev-flco-06 · Randomized controlled trial(s) · open randomized controlled pilot trial, three arms · n = 40
   There were no significant changes in stool frequency or stool consistency for any of the groups.
   
   Who: adults with irritable bowel syndrome
   Effect: no significant change in stool frequency or consistency in any group; symptom severity vs control P = 0.12 (whole) and 0.10 (ground)
   Certainty: pilot, open, 31 completers; underpowered
   J Hum Nutr Diet, 2012 · checked 2026-10-06 · we read the abstract
7. ev-flco-07 · Randomized controlled trial(s) · randomized double-blind crossover trial, 3-week periods · n = 43
   During the test yoghurt period, defecation frequency was higher (8.0 vs 7.1 times/week, P=0.011), defecation was easier (on the scale 0-3, 1.3 vs 1.5, P=0.010), and there was a tendency towards softer stools (on the scale 0-3, 2.1 vs 2.2, P=0.059) compared with the control yoghurt period.
   
   Who: free-living elderly subjects with self-reported constipation, mean age 76
   Effect: defecation frequency 8.0 vs 7.1 times/week, P = 0.011; easier defecation
   Certainty: linseed was one of three active ingredients, so its own share is unknown; industry funded
   Eur J Clin Nutr, 2007 · checked 2026-10-06 · we read the abstract
8. ev-flco-08 · Randomized controlled trial(s) · randomized crossover trial, 4 weeks · n = —
   Bowel movements per week increased by 30% while flaxseed was consumed (P < 0.05).
   
   Who: healthy young adults
   Effect: bowel movements per week +30%, P < 0.05
   Certainty: not a constipated population; sample size not stated in abstract, full text not in PMC
   Am J Clin Nutr, 1995 · checked 2026-10-06 · we read the abstract
9. ev-flco-09 · Randomized controlled trial(s) · parallel randomized controlled trial, active comparator, 4 days · n = 70
   The mean and standard deviation of the bowel movement frequency for the patients in the intervention and control groups are 1.86±1.08 and 1.6±0.65, respectively. Thus, there is no significant difference between the groups (P=0.234).
   
   Who: acute myocardial infarction patients in ICU without chronic constipation, Iran
   Effect: bowel movements 1.86 vs 1.6 over five days, P = 0.234; time to first defecation shorter with magnesium hydroxide, P = 0.023
   Certainty: prevention in bedbound patients, short, open label
   Caspian Journal of Internal Medicine, 2024, 15(2):234 (flaxseed powder vs magnesium hydroxide in ICU) · checked 2026-10-06 · we read the fulltext
10. ev-flco-10 · Meta-analysis or systematic review · systematic review of RCTs with USPSTF-style grading · n = 41 trials
    There was good evidence (grade A recommendation) for the use of the osmotic laxative polyethylene glycol (PEG) and the stimulant senna; moderate evidence (grade B) for psyllium, SupraFiber, magnesium salts, stimulants (bisacodyl and sodium picosulfate), fruit-based laxatives (kiwi, mango, prunes, and ficus), and yogurt with galacto-oligosaccharide/prunes/linseed oil; and insufficient evidence (grade I) for polydextrose, inulin, and fructo-oligosaccharide.
    
    Who: adults with chronic constipation in randomized trials of at least 4 weeks
    Effect: moderate evidence (grade B) for the combination yogurt; good evidence (grade A) only for PEG and senna
    Certainty: no pooled estimate; linseed not graded alone
    Am J Gastroenterol, 2021 · checked 2026-10-06 · we read the abstract
11. ev-flco-11 · Meta-analysis or systematic review · systematic review of RCTs with USPSTF-style grading · n = 1 trial
    In another study, flaxseed oil, olive oil, and mineral oil (control) all significantly reduced the frequency of symptom scores (ROME criteria) from baseline (62).
    
    Who: constipated patients with chronic kidney disease in a randomized trial
    Effect: flaxseed oil, olive oil and mineral oil all reduced symptom scores from baseline
    Certainty: flaxseed oil lacks the seed fiber and mucilage; one trial
    Am J Gastroenterol, 2021 · checked 2026-10-06 · we read the fulltext
12. ev-flco-12 · Observational data · single-arm before-after clinical study with healthy reference group · n = 60
    After flax seeds treatment, the total Wexner score in constipation patients decreased from 14.38 ± 3.84 to 7.28 ± 3.29 (P<0.05, Figure 1A), and the frequency of defecation (times/week) increased significantly from 3.79 ± 2.24 to 6.22 ± 2.44 (P<0.05, Figure 1B).
    
    Who: elderly patients with functional constipation in China, mean age 68.7
    Effect: defecation frequency 3.79 to 6.22 times/week; Wexner score 14.38 to 7.28; no adverse reactions
    Certainty: no randomized control; responder rate self-defined
    J Multidiscip Healthc, 2022 · checked 2026-10-06 · we read the fulltext
13. ev-flco-13 · Position of an expert body · agency consumer fact sheet · n = —
    Higher doses of flaxseed or flaxseed oil supplements may cause uncomfortable digestive symptoms like bloating, fullness, and diarrhea.
    
    Who: adults using flaxseed or flaxseed oil supplements
    Effect: digestive side effects at higher doses
    Certainty: no dose threshold given
    NIH National Center for Complementary and Integrative Health, Flaxseed and Flaxseed Oil (updated February 2025) · checked 2026-10-06 · we read the section
14. ev-flco-14 · Position of an expert body · agency consumer fact sheet · n = —
    Some studies of the use of flaxseed during pregnancy suggest it is possibly unsafe, but the evidence is not conclusive.
    
    Who: pregnant women
    Effect: possibly unsafe in pregnancy
    Certainty: underlying studies not named on the page
    NIH National Center for Complementary and Integrative Health, Flaxseed and Flaxseed Oil (updated February 2025) · checked 2026-10-06 · we read the section
15. ev-flco-15 · Position of an expert body · agency consumer fact sheet · n = —
    There are theoretical reasons to suspect that flaxseed or flaxseed oil might interact with other drugs, such as anticoagulant or antiplatelet drugs.
    
    Who: people taking anticoagulant or antiplatelet drugs
    Effect: theoretical interaction
    Certainty: theoretical, no clinical cases cited
    NIH National Center for Complementary and Integrative Health, Flaxseed and Flaxseed Oil (updated February 2025) · checked 2026-10-06 · we read the section
16. ev-flco-16 · Laboratory or animal data · case report · n = 1
    Hypersensitivity to linseeds has been infrequently described, and we report a case of anaphylaxis induced by linseed ingestion in a 39-year-old woman.
    
    Who: one adult woman
    Effect: anaphylaxis with linseed-specific IgE
    Certainty: single case; frequency unknown
    Allergol Immunopathol (Madr), 2003 · checked 2026-10-06 · we read the abstract
17. flx-c1-08 · Randomized controlled trial(s) · randomized controlled dose-ranging trial · n = 34
    Plasma ALA concentrations increased with all flaxseed doses (P < 0.01), except the 20 g/day dose (P = 0.10), yet there was no significant dose-dependent response (P = 0.81). Only with the 30 g/day diet were n-3 polyunsaturated fatty acids (P = 0.007), and eicosapentaenoic acid (EPA) (P = 0.047) increased from baseline values. ... Flaxseed was well tolerated, even at the highest dose, as there were no reported adverse events, changes in cholesterol, platelet aggregation or urinary 11-dehydro-thromboxane B2.
    
    Who: 34 healthy adults aged 18 to 49, one muffin daily with 10, 20, 30 or 40 g milled flaxseed, 30 days
    Effect: plasma ALA up at 10, 30 and 40 g/day (P < 0.01), not at 20 g (P = 0.10), no dose-response; EPA up only at 30 g/day (P = 0.047); enterolignans up at all doses; no adverse events, no cholesterol change
    Certainty: 8 to 9 people per arm; 30 days; the missing effect at 20 g/day suggests noise rather than a true dose gap
    Eur J Nutr, 2016 · checked 2026-09-23 · we read the abstract
18. flx-c1-10 · Observational data · non-randomized controlled feeding study in healthy women with food chemistry analyses · n = —
    Flaxseed raised alpha-linolenic acid and long-chain n-3 fatty acids in both plasma and erythrocyte lipids, as well as raising urinary thiocyanate excretion 2.2-fold. Flaxseed also lowered serum total cholesterol by 9% and low-density-lipoprotein-cholesterol by 18%. ... Cyanogenic glycosides (linamarin, linustatin, neolinustatin) were highest in extracted flaxseed mucilage but were not detected in baked muffins containing 150 g flaxseed/kg.
    
    Who: healthy women eating 1.8 oz (50 g) raw ground flaxseed a day (12 to 13 percent of energy) for 4 weeks; test meals and baked muffins analyzed
    Effect: urinary thiocyanate 2.2-fold; total cholesterol -9%, LDL -18%; postprandial glucose -27% with 50 g carbohydrate from flaxseed or 25 g mucilage; cyanogenic glycosides highest in extracted mucilage, not detected in baked muffins
    Certainty: 1993 single-group feeding study, no randomized control, sample size not in the abstract; thiocyanate rise shows cyanide exposure at 50 g/day raw seed even though the authors judged it safe
    Br J Nutr, 1993 · checked 2026-09-23 · we read the abstract
19. flx-c1-16 · Position of an expert body · European Commission regulation, recital and annex · n = —
    When ground linseed as such is consumed, the bioavailability of hydrocyanic acid and human exposure levels to it are higher than when whole linseeds are consumed or when they are heat-treated.
    
    Who: linseed placed on the EU market, including ground, milled, cracked and chopped seed sold to consumers
    Effect: Regulation (EU) 2022/1364 amends the contaminant limits for hydrocyanic acid in linseed (Annex rows 8.3.1 and 8.3.2) and exempts small consumer packs labeled Only to be used for cooking and baking. Do not consume raw!; the numeric mg/kg limits did not render in our fetch
    Certainty: a legal limit reflects the regulator's exposure assessment (EFSA ARfD 20 microgram cyanide per kg body weight), not a trial; the sentence was confirmed on the page by two independent fetches but the numeric limits could not be read from the table rendering
    Commission Regulation (EU) 2022/1364, Official Journal, 2022 · checked 2026-09-23 · we read the section
20. flxs-r5-07 · Observational data · systematic review of documented interaction and contraindication reports · n = 1491 pairs
    Flaxseed, echinacea, and yohimbe had the largest number of documented contraindications.
    
    Who: 1,491 unique pairs of herb or supplement with drug, 213 supplement entities and 509 medications, from 85 primary studies, six books and two web sites
    Effect: 152 contraindications identified in total, most often gastrointestinal (16.4%), neurological (14.5%) and renal or genitourinary (12.5%); flaxseed among the three entities with the most
    Certainty: Documentation counts, not measured risk. A contraindication reaches this list because somebody wrote it down, and the review counts entries rather than events. It also covers supplement doses, not a spoonful of seed.
    Int J Clin Pract, 2012 · checked 2026-09-24 · we read the abstract
21. flxs-r5-08 · Observational data · narrative and meta-analytic review of case reports · n = not stated in abstract
    Considering the worldwide consumption of those seeds, the number of published articles on the subject was relatively small and was mainly as case reports rather than large series.
    
    Who: English literature from January 1930 to March 2016 on hypersensitivity to sesame, sunflower, poppy, pumpkin, flax and mustard seed
    Effect: sesame allergy prevalence estimated at 0.1 to 0.2%; for the other five seeds, including flaxseed, only case reports; routine diagnostic tests of limited reliability and challenge tests needed
    Certainty: Not a prevalence study. The honest content is that reactions exist, can be severe, and that nobody has counted them for flaxseed.
    Allergy, 2016 · checked 2026-09-24 · we read the abstract