Flaxseed and flaxseed oil

What it is

Flaxseed is the seed of the flax plant, used as a food for its alpha-linolenic acid, its lignans and its dietary fibre. Flaxseed oil is the oil pressed from the seed, and it carries the alpha-linolenic acid but not the fibre or the lignans, which matters because the three products behave differently in trials. Alpha-linolenic acid is the omega-3 fatty acid found in plant oils including flaxseed, soybean and canola oil. Flaxseed also contains the cyanogenic glycosides linustatin, neolinustatin and linamarin, which is why food safety agencies have assessed it.

What the research says

The state of the evidence is that whole ground flaxseed has a measurable but modest effect on blood lipids and blood pressure in meta-analyses of randomised trials, while flaxseed oil does not lower lipids, and neither reliably touches hot flushes. A 2021 dose-response meta-analysis of 31 randomised trials found whole flaxseed lowered total and LDL cholesterol and triglycerides but had no effect on HDL cholesterol, C-reactive protein or body measurements; flaxseed oil lowered inflammatory markers rather than lipids. A 2024 meta-analysis of five trials in hypertension found systolic and diastolic blood pressure fell, but with very large heterogeneity between studies. A 188-woman phase III trial found flaxseed lignans no better than placebo for hot flushes. On the harm side, flaxseed is an established cause of anaphylaxis in a small number of reported cases, its cyanogenic glycosides have prompted formal agency risk assessments, and gastrointestinal effects from the fibre are common.

Evidence grade: Limited evidence.

What goes wrong

In that trial, gastrointestinal effects were reported in both arms and were attributed to the fibre content. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 188 women enrolled.
• Who: postmenopausal women with or without breast cancer.
• How long: 6 weeks.
• Result: abdominal distension, flatulence, diarrhoea and nausea reported in both arms.
• Funding: not stated in the abstract.

The bars were fairly well tolerated, with both groups reporting gastrointestinal effects, likely due to the fiber content.

The only side effect that differed significantly between arms in that trial was grade 1 pruritus, which was commoner on placebo. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 188 women enrolled.
• Who: postmenopausal women with or without breast cancer.
• How long: 6 weeks.
• Result: grade 1 pruritus 8% on placebo versus 1% on flaxseed.
• Funding: not stated in the abstract.

Only one side effect was significantly different between groups, grade 1 pruritis, which was more common in the placebo group (8% versus 1%).

Flaxseed causes anaphylaxis in a small number of published cases, including in people with no history of allergy. (Source 2)

• Case series, Very low certainty.
• Size: 2 new cases reported, plus 13 cases identified in the authors' literature review.
• Who: an 81-year-old man and a 9-year-old boy, plus previously reported cases mostly in middle-aged adults.
• How long: reactions within minutes of ingestion.
• Result: case 1 had hypotension, tachycardia, bronchoconstriction, widespread rash, acute tryptase 17.9 ug/L and flaxseed-specific IgE 18.5 kUA/L and required epinephrine.
• Funding: not stated.

In our literature review, a total of 13 cases were found, primarily involving middle-aged adults, the majority of them with no history of atopy described.

The same authors describe flaxseed as an emerging allergen whose identified allergens are linked to severe reactions. (Source 3)

• Case series, Very low certainty.
• Size: 2 cases plus a literature review.
• Who: people with flaxseed allergy.
• How long: not applicable.
• Result: no incidence rate reported; severity described as severe.
• Funding: not stated.

Flaxseed is an emerging allergen, and a detailed clinical history is crucial for diagnosis. Flaxseed allergens identified are associated with severe reactions.

POSITION: Austria's food safety agency AGES (1 December 2017) states that flaxseed carries the cyanogenic glycosides linustatin, neolinustatin and linamarin, from which hydrocyanic acid can be released. (Source 4)

• Official position, Certainty not rated.
• Size: not applicable; the page cites a human study of blood cyanide after different foods.
• Who: general population.
• How long: not applicable.
• Result: blood cyanide rises only slightly after ground flaxseed because flaxseed contains little beta-glucosidase.
• Funding: Austrian government agency.

The cyanogenic glycosides linustatin, neolinustatin and linamarin are mainly found in flaxseed.

POSITION: AGES (December 2017) judged that up to 30 g of ground flaxseed per meal poses a risk that can be largely ruled out for adults and adolescents from 13, but that an adverse effect in children cannot be ruled out. (Source 4)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: adults, adolescents from 13, and children.
• How long: per meal.
• Result: no risk estimate given; risk 'largely ruled out' for over-13s at up to 30 g ground flaxseed per meal, not ruled out for children.
• Funding: Austrian government agency.

At normal consumption levels of up to 30 g of ground flaxseed per meal, a risk to the health of adolescents from the age of 13 and adults can be largely ruled out.

POSITION: the EFSA CONTAM Panel (2019) applied an acute reference dose of 20 micrograms cyanide per kg body weight to all dietary sources of cyanogenic glycosides, and found the ARfD exceeded at the 95th percentile in some child and adolescent surveys. (Source 5)

• Official position, Certainty not rated.
• Size: EU dietary exposure surveys.
• Who: all EU age groups, with children and adolescents highlighted.
• How long: acute exposure.
• Result: mean acute exposures did not exceed the ARfD in any age group; at the 95th percentile the ARfD was exceeded up to about 2.5-fold in some child and adolescent surveys.
• Funding: European Food Safety Authority.

In the present opinion, the CONTAM Panel concluded that this ARfD is applicable for acute effects of CN regardless the dietary source.

POSITION: NIH Office of Dietary Supplements records that no tolerable upper intake level was set for any omega-3, and that high intakes of long-chain omega-3s can lengthen bleeding time. (Source 6)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: general population.
• How long: several weeks or longer at high doses.
• Result: 900 mg/day EPA plus 600 mg/day DHA or more for several weeks might reduce immune function; 2-15 g/day EPA and/or DHA might increase bleeding time.
• Funding: US government agency page.

The IOM did not establish a UL for any omega-3s, although it noted that high doses of DHA and/or EPA (900 mg/day of EPA plus 600 mg/day DHA or more for several weeks) might reduce immune function due to suppression of inflammatory responses.

What the evidence supports

A dose-response meta-analysis of randomised trials found whole flaxseed lowered total cholesterol, LDL cholesterol and triglycerides in patients with dyslipidaemia-related disease. (Source 7)

• Meta-analysis, Certainty not rated.
• Size: 1,698 participants across 31 randomised controlled trials.
• Who: patients with dyslipidaemia-related diseases.
• How long: varied; subgroup analysis found the effect held irrespective of intervention time.
• Result: total cholesterol -11.85 mg/dl (95% CI -20.12 to -3.57, P = 0.005); LDL-C -10.51 mg/dl (95% CI -14.96 to -6.06, P < 0.001); triglycerides -19.77 mg/dl (95% CI -33.61 to -5.94, P = 0.005); apolipoprotein B -5.73 mg/dl (95% CI -7.53 to -3.93, P < 0.001); weight -0.40 kg (95% CI -0.76 to -0.05, P = 0.027)
• Funding: not stated in the abstract; no GRADE certainty rating reported.

Whole flaxseed supplementation significantly reduced TC (− 11.85 mg/dl, 95% CI − 20.12 to − 3.57, P = 0.005), LDL-C (− 10.51 mg/dl, 95% CI − 14.96 to − 6.06, P < 0.001), TG (− 19.77 mg/dl, 95% CI − 33.61 to − 5.94, P = 0.005), apolipoprotein B (− 5.73 mg/dl, 95% CI − 7.53 to − 3.93, P < 0.001), TC/HDL-C (− 0.10, 95% CI − 0.19 to − 0.003, P = 0.044) and weight (− 0.40 kg, 95% CI − 0.76 to − 0.05, P = 0.027)

A 2024 meta-analysis of five randomised trials in hypertensive patients found flaxseed supplementation lowered systolic and diastolic blood pressure. (Source 8)

• Meta-analysis, Low certainty.
• Size: 5 randomised controlled trials.
• Who: patients with hypertension.
• How long: varied; subgroup analyses separated trials over and under 12 weeks.
• Result: systolic BP WMD -8.64 mmHg (95% CI -15.41 to -1.87; p <= 0.001); diastolic BP WMD -4.87 mmHg (95% CI -8.37 to -1.37; p = 0.006)
• Funding: not stated in the abstract; risk of bias assessed with the Cochrane tool; only five trials pooled.

Limit of this finding: The paper's own figures do not agree with each other. A pooled fall of 8.64 mmHg with a 95% confidence interval reaching to 1.87 mmHg is only just outside no effect, which does not match the p ≤ 0.001 printed beside it; a confidence interval that wide corresponds to a p-value of roughly 0.01. The real uncertainty is wide, the pooled result comes from only five small trials, and the authors themselves present flaxseed as a possible adjuvant, so the size of the fall should be treated as very uncertain rather than as a reliable 8.64 mmHg.

The results show that there is a significant decrease in systolic BP (WMD, −8.64 mmHg; 95% CI, −15.41 to −1.87; p ≤ 0.001) and diastolic BP (WMD, −4.87 mmHg; 95% CI, −8.37 to −1.37; p = 0.006) of patients with hypertension as compared to control groups.

The review's own conclusion is that whole flaxseed and lignan extracts account for the lipid lowering, while flaxseed oil acts mainly on inflammation. (Source 9)

• Meta-analysis, Certainty not rated.
• Size: 1,698 participants across 31 randomised controlled trials.
• Who: patients with dyslipidaemia-related diseases.
• How long: varied between trials.
• Result: no new numbers in the conclusion; the pooled estimates are in the paper's Results.
• Funding: not stated in the abstract.

Limit of this finding: The paper's own conclusion also says the lipid and weight lowering was significant when whole flaxseed was consumed at "doses < 30 mg/d". That figure is an error in the published paper: the Results section of the same abstract puts the threshold at 30 g per day of whole flaxseed, which is a thousand times more, and 30 mg of flaxseed is far too little to have any measurable effect. Read the threshold as the Results state it, 30 g a day or less, and do not read either number as a dose to take.

Of these, whole flaxseed and lignans play an important role in reducing blood lipid, while flaxseed oil mainly plays in anti-inflammatory.

What the evidence does not support

In the same meta-analysis, flaxseed oil did not lower blood lipids, unlike whole flaxseed and lignan extracts. (Source 7)

• Meta-analysis, Certainty not rated.
• Size: subset of the 31 trials using flaxseed oil.
• Who: patients with dyslipidaemia-related diseases.
• How long: varied.
• Result: no lipid-lowering effect for flaxseed oil; IL-6 -0.35 pg/ml (P = 0.033) and hs-CRP -1.54 mg/l (P = 0.004)
• Funding: not stated in the abstract.

Although flaxseed oil supplementation had no such lowering-effect on lipid, meta-analysis revealed its lowering-effect on IL-6 (− 0.35 pg/ml, P = 0.033) and hs-CRP (− 1.54 mg/l, P = 0.004).

Pooled across all flaxseed products, there was no effect on apolipoprotein A, HDL cholesterol, C-reactive protein or body measurements. (Source 7)

• Meta-analysis, Certainty not rated.
• Size: 1,698 participants across 31 randomised controlled trials.
• Who: patients with dyslipidaemia-related diseases.
• How long: varied.
• Result: null for apo A, HDL-C, hs-CRP, CRP and anthropometric indices.
• Funding: not stated in the abstract.

The present meta-analysis revealed that flaxseed consumption had an overall beneficial effect on serum TC, LDL-C, TG, apo B and IL-6 in patients with dyslipidemia related diseases, but not on apo A, HDL-C, hs-CRP, CRP and anthropometric indices.

A phase III randomised placebo-controlled trial found a flaxseed bar providing 410 mg of lignans no better than placebo for hot flushes. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 188 women enrolled.
• Who: postmenopausal women with or without breast cancer.
• How long: one baseline week then 6 weeks of treatment.
• Result: mean hot flash score fell 4.9 on flaxseed versus 3.5 on placebo (p = .29); a little over a third of women in each group had a 50% reduction.
• Funding: North Central Cancer Treatment Group cooperative-group trial; funding not stated in the abstract.

Mean hot flash score was reduced 4.9 in the flaxseed group and 3.5 in the placebo group (p=.29). In both groups, a little over a third of the women received a 50% reduction in their hot flash score.

The authors of that trial state their results do not support using 410 mg of lignans for hot flushes. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 188 women enrolled.
• Who: postmenopausal women with or without breast cancer.
• How long: 6 weeks of treatment.
• Result: primary endpoint negative, p = .29.
• Funding: not stated in the abstract.

The results of this trial do not support the use of 410 mg of lignans for the reduction of hot flashes.

Where the research disagrees

How much concern the cyanogenic glycosides in flaxseed deserve

• AGES, Austrian Agency for Health and Food Safety, 1 December 2017, agency risk assessment citing a human study of blood cyanide after different cyanogenic foods: At normal consumption levels of up to 30 g of ground flaxseed per meal, a risk to the health of adolescents from the age of 13 and adults can be largely ruled out. In the case of children, the consumption of up to 30 g of flaxseed may have an adverse effect on their health, but this cannot be ruled out according to the current state of knowledge. An adverse health effect from the consumption of whole, unground linseed or linseed oil is considered to be excluded. (Source 4)
• EFSA Panel on Contaminants in the Food Chain, 2019, EU-wide acute dietary exposure assessment against an acute reference dose of 20 micrograms cyanide per kg body weight: At the 95th percentile, the ARfD was exceeded up to about 2.5‐fold in some surveys for children and adolescent age groups. (Source 5)

How much

• Reference intake: There is no RDA for omega-3 fatty acids. NIH Office of Dietary Supplements records that the Institute of Medicine had insufficient data to set an Estimated Average Requirement and set Adequate Intakes instead, and that from age 1 onward the AI applies only to alpha-linolenic acid, the omega-3 that flaxseed supplies, because ALA is the only essential omega-3. No body sets a reference intake for flaxseed itself. (Source 10)
• Upper limit: NIH Office of Dietary Supplements states the Institute of Medicine did not establish a Tolerable Upper Intake Level for any omega-3, while noting that high doses of EPA and DHA might reduce immune function and that 2-15 g/day might increase bleeding time. No UL exists for alpha-linolenic acid or for flaxseed. (Source 6)
• Studied: Trials pooled in the 2021 dose-response meta-analysis gave whole flaxseed, lignan extracts or flaxseed oil; the lipid effect of whole flaxseed was more pronounced at doses of 30 g/day or less than above 30 g/day. (Source 7)
• Studied: The phase III NCCTG N08C7 trial gave postmenopausal women one bar a day providing 410 mg of lignans for 6 weeks. (Source 1)
• Studied: Austria's AGES (December 2017) frames its assessment around consumption of up to 30 g of ground flaxseed per meal. (Source 4)

A common belief, and what the research shows

The belief: Flaxseed oil is just a convenient version of ground flaxseed and does the same thing.

What the research shows: They separate in the trial data. In the 2021 dose-response meta-analysis of 31 randomised trials, "Whole flaxseed supplementation significantly reduced TC (− 11.85 mg/dl, 95% CI − 20.12 to − 3.57, P = 0.005), LDL-C (− 10.51 mg/dl, 95% CI − 14.96 to − 6.06, P < 0.001)", while for the oil the authors wrote "Although flaxseed oil supplementation had no such lowering-effect on lipid, meta-analysis revealed its lowering-effect on IL-6 (− 0.35 pg/ml, P = 0.033) and hs-CRP (− 1.54 mg/l, P = 0.004)." The oil carries the alpha-linolenic acid but not the fibre or lignans.

Questions and answers

What is it?

Flaxseed is the seed of the flax plant, eaten whole, ground or as pressed oil. It is used as a food for three separate components: the omega-3 fatty acid alpha-linolenic acid, plant compounds called lignans, and dietary fibre. Flaxseed oil contains the fatty acid but not the fibre or lignans, and the three forms behave differently in trials. (Source 11)

What does it do in the body?

Alpha-linolenic acid is an essential fatty acid, and the body can convert a small part of it into the longer-chain omega-3s EPA and DHA. NIH ODS states that this conversion, which happens mainly in the liver, is very limited, with reported rates of less than 15 percent, so flaxseed is not a substitute for the omega-3s in oily fish. In trials, whole flaxseed lowers total and LDL cholesterol modestly, and pooled trials in hypertension show a fall in blood pressure. (Source 12)

Is it good or bad for you?

Whole ground flaxseed has modest measured benefits for cholesterol and blood pressure in meta-analyses of randomised trials, though certainty is not formally graded and heterogeneity between studies is large. The same pooled analysis found no effect on HDL cholesterol, C-reactive protein or body measurements, and flaxseed oil did not lower lipids at all. The harms are gastrointestinal effects from the fibre, anaphylaxis in a small number of reported cases, and the cyanogenic glycosides that food safety agencies have assessed and judged a concern chiefly for children. (Source 7)

How do you get more of it?

Alpha-linolenic acid, the omega-3 in flaxseed, is found in flaxseed (linseed) oil and in soybean and canola oils. Trials in the lipid meta-analysis used whole flaxseed, lignan extracts or flaxseed oil, with whole-flaxseed doses of 30 g a day or less showing the clearest lipid effect. This is a description of what studies used, not a recommendation. (Source 13)

If it is harmful, what reduces it?

The situation where flaxseed is clearly harmful is allergy, and there the only measure in the literature is avoidance. The authors of a 2024 case series note that giving clear avoidance advice is difficult because no clinical cross-reactivity with other foods has been reported. For the cyanogenic glycosides, AGES notes that whole unground linseed and linseed oil are not considered a concern, and that grinding is what releases them. (Source 3)

Why might someone be low in it or missing it?

Alpha-linolenic acid intake varies with diet, since it comes from plant oils, seeds and nuts. NIH ODS states that classical essential fatty acid deficiency in healthy people in the United States is virtually nonexistent, because the body releases essential fatty acids from fat stores when dietary fat is restricted. Where clinical deficiency has been documented it was in patients on parenteral nutrition lacking polyunsaturated fats. (Source 14)

Which whole foods contain it or feed it?

Foods supplying alpha-linolenic acid include flaxseed (linseed) oil, soybean oil and canola oil, and NIH ODS notes that chia seeds and walnuts also contain it. Cold-water fatty fish such as salmon, mackerel, tuna, herring and sardines supply the longer-chain omega-3s EPA and DHA instead, which is a different thing; leaner fish such as bass, tilapia and cod, and shellfish, contain lower levels of those. (Source 13)

What happens if you do not have it?

Flaxseed itself is not required; what is essential is alpha-linolenic acid, which many other foods supply. NIH ODS records that a deficiency of essential fatty acids, omega-3 or omega-6, can cause rough scaly skin and dermatitis, and that tissue DHA falls when omega-3 intake is deficient. It also states there are no known cut-off concentrations below which function is impaired. (Source 14)

How can you test for it?

There is no test for flaxseed intake. Omega-3 status can be measured in plasma or serum phospholipids, or more stably in red cell membranes as the omega-3 index, but NIH ODS notes most clinicians do not assess it and that experts have not established normal ranges. Plasma values also shift with the most recent meal, so they do not reflect long-term intake. Flaxseed allergy, by contrast, is diagnosed with specific IgE and skin prick-to-prick testing. (Source 14)

References

1. Menopause. A Phase III, Randomized, Placebo-Controlled, Double-Blind Trial of Flaxseed for the Treatment of Hot Flashes: NCCTG N08C7. 2012. PMID 21900849, DOI 10.1097/gme.0b013e318223b021. Read the source
2. Porto Biomedical Journal. Flaxseed anaphylaxis: an emerging allergen - literature review of reported cases. 2024. PMID 39464209, DOI 10.1097/j.pbj.0000000000000265. Read the source
3. Porto Biomedical Journal. Flaxseed anaphylaxis: an emerging allergen - abstract / key messages. 2024. PMID 39464209, DOI 10.1097/j.pbj.0000000000000265. Read the source
4. AGES - Austrian Agency for Health and Food Safety (page dated 1 December 2017). Cyanogenic glycosides in flaxseed. 2017. Read the source
5. EFSA Journal (EFSA Panel on Contaminants in the Food Chain). Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels. 2019. DOI 10.2903/j.efsa.2019.5662. Read the source
6. NIH Office of Dietary Supplements. Omega-3 Fatty Acids - Fact Sheet for Health Professionals (Health Risks From Excessive Omega-3s). 2025. Read the source
7. Nutrition & Metabolism. Comparisons of the effects of different flaxseed products consumption on lipid profiles, inflammatory cytokines and anthropometric indices in patients with dyslipidemia related diseases: systematic review and a dose-response meta-analysis of randomized controlled trials - Abstract, Results. 2021. PMID 34635132, DOI 10.1186/s12986-021-00619-3. Read the source
8. Clinical Nutrition Research. Flaxseed Lowers Blood Pressure in Hypertensive Subjects: A Meta-Analysis of Randomized Controlled Trials. 2024. DOI 10.7762/cnr.2024.13.4.295. Read the source
9. Nutrition & Metabolism. Comparisons of the effects of different flaxseed products consumption on lipid profiles, inflammatory cytokines and anthropometric indices in patients with dyslipidemia related diseases - Abstract, Conclusions. 2021. PMID 34635132, DOI 10.1186/s12986-021-00619-3. Read the source
10. NIH Office of Dietary Supplements. Omega-3 Fatty Acids - Fact Sheet for Health Professionals (Recommended Intakes). 2025. Read the source
11. Nutrition & Metabolism. Comparisons of the effects of different flaxseed products consumption on lipid profiles, inflammatory cytokines and anthropometric indices in patients with dyslipidemia related diseases - Abstract, Background and Methods. 2021. PMID 34635132, DOI 10.1186/s12986-021-00619-3. Read the source
12. NIH Office of Dietary Supplements. Omega-3 Fatty Acids - Fact Sheet for Health Professionals. 2025. Read the source
13. NIH Office of Dietary Supplements. Omega-3 Fatty Acids - Fact Sheet for Health Professionals. 2025. Read the source
14. NIH Office of Dietary Supplements. Omega-3 Fatty Acids - Fact Sheet for Health Professionals (assessing omega-3 status and omega-3 deficiency). 2025. Read the source