Flax seeds: real blood pressure numbers, at supplement doses

Thirty-three randomized trials with 2,427 participants, heterogeneity above 90 percent and whole seed, oil and lignan extract pooled as one intervention, put flaxseed supplementation at 3.19 mmHg lower systolic blood pressure (95% CI 4.15 to 2.24 lower, p < 0.001). Diastolic pressure fell 2.61 mmHg (95% CI 3.27 to 1.94, p < 0.001). That is a genuine effect at trial doses, and trial doses start where the larger reductions sit, at 30 g a day and beyond 20 weeks, which is several spoonfuls and not a sprinkle on porridge. At the worst-case cyanide level EFSA found, about 11 g of raw ground seed in one sitting reaches its acute limit for an average adult, and about 4 g, a teaspoon, takes a toddler past it, though EFSA judges that limit likely over-conservative for linseed. Inflammation markers moved in the same direction. Across 54 trials and 3,000 adults in 12 countries, with heterogeneity of 83 to 93 percent, C-reactive protein fell by a standardized mean difference of 0.46 (95% CI 0.70 to 0.23, p < 0.001). Interleukin 6 fell by 0.64 (95% CI 1.13 to 0.16, p = 0.010), and tumor necrosis factor alpha did not move (p = 0.467). These are blood markers and not symptoms or events, and a standardized mean difference carries no familiar unit behind it. We hold no USDA composition record for flax seeds in this cohort, so there are no per-100 g numbers on this page.

As people eat it, per 3.5 oz (100 g)

What your body absorbs

• In the human study EFSA relies on, 1.1 oz (30.9 g) of linseed carrying 6.8 mg of cyanide raised blood cyanide far less than cassava with the same amount, a mean peak of 6.40 against 16.95 µM. EFSA explains this by weaker seed enzymes and judges its acute limit likely over-conservative for foods such as linseed. — human volunteers in the bioavailability study EFSA relies on Evidence E source
  30.9 g linseed with 6.8 mg cyanide gave a mean blood cyanide peak of 6.40 µM against 16.95 µM for cassava with the same dose; cyanide released more slowly because of lower beta-glucosidase activity; ARfD likely over-conservative for linseed
• In a 30-day trial in 34 healthy adults eating a muffin with 10, 20, 30 or 1.4 oz (40 g) of milled flaxseed a day, no adverse events were reported at any dose and platelet aggregation did not change. Only the 30 g dose raised blood EPA. — healthy, younger adults aged 18 to 49 years (n = 34) Evidence B source
  10, 20, 30 or 40 g/day milled flaxseed in a muffin for 30 days: no reported adverse events and no change in platelet aggregation at any dose; only 30 g/day raised EPA

What cooking and storage change

• Since 2022 EU law caps hydrocyanic acid at 150 mg per kg in whole or ground linseed sold to consumers and 250 mg per kg in other linseed. It gives the reason that ground linseed eaten raw releases more cyanide than whole or heat-treated seed. — linseed placed on the EU market Evidence E source
  Hydrocyanic acid at most 150 mg/kg in linseed sold to the final consumer and 250 mg/kg in other linseed; stricter because ground raw linseed releases more; exemption for small packs labeled 'Only to be used for cooking and baking. Do not consume raw!'

What it does to your health

• Pooling 33 randomized trials with 2,427 participants, flaxseed supplementation lowered systolic blood pressure by 3.19 mmHg (95% CI 4.15 to 2.24 lower) and diastolic by 2.61 mmHg, with larger effects above 30 g a day and beyond 20 weeks. — 2,427 participants in 33 trials with 43 treatment arms, including people with hypertension and BMI 25 to 30 (n = 2427) Evidence A source
  Systolic WMD -3.19 mmHg (95% CI -4.15 to -2.24), diastolic WMD -2.61 mmHg (95% CI -3.27 to -1.94), both p < 0.001; I2 92.5% and 94.1%
• In 16 randomized trials with 1,136 people who had diabetes, flaxseed cut weight, waist circumference and triglycerides, and changed neither blood pressure nor LDL cholesterol. — 1,136 participants with diabetes across 16 randomized controlled trials, GRADE assessed (n = 1136) Evidence A source
  Weight SMD 0.81 (p = 0.028), waist circumference SMD 1.54 (p = 0.002), triglycerides SMD 0.56 (p = 0.016); BMI, systolic and diastolic pressure, total cholesterol, LDL and HDL all non-significant
• In six randomized trials of 309 people with coronary artery disease, flaxseed lowered fasting blood glucose by 8.35 mg/dL and hs-CRP by 1.35 mg/L, and left weight and most lipids unchanged. — 309 randomized participants with coronary artery disease, 153 on flaxseed and 156 on placebo, trials published 2017 to 2022 (n = 309) Evidence A source
  Fasting glucose WMD -8.35 mg/dL (95% CI -15.01 to -1.69, p = 0.01), hs-CRP WMD -1.35 mg/L (95% CI -1.93 to -0.77, p < 0.01), triglycerides -18.39 mg/dL only after sensitivity analysis
• Across 54 randomized trials and 3,000 adults in 12 countries, flaxseed lowered C-reactive protein (SMD 0.46) and interleukin 6 (SMD 0.64), while tumor necrosis factor alpha did not move. — 3,000 adults in 54 randomized controlled trials from 12 countries, searched to February 2024 (n = 3000) Evidence A source
  CRP SMD -0.46 (95% CI -0.70 to -0.23, p < 0.001), IL-6 SMD -0.64 (95% CI -1.13 to -0.16, p = 0.010), TNF-alpha SMD -0.17 (p = 0.467)

Safety, limits and interactions

• Flaxseed heads the list of supplements with documented contraindications: in a review of 1,491 supplement and drug pairs, flaxseed, echinacea and yohimbe carried the largest number of them. — 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 (n = 1491 pairs) Evidence C source
  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
• Flaxseed is one of six edible seeds with documented hypersensitivity reactions, and the literature on it is case reports rather than series, so no prevalence for flaxseed allergy can be stated. — English literature from January 1930 to March 2016 on hypersensitivity to sesame, sunflower, poppy, pumpkin, flax and mustard seed (n = not stated in abstract) Evidence C source
  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
• The limit that bounds cyanogenic foods is an EFSA acute reference dose of 20 micrograms of cyanide per kilogram of body weight, which for a 154 lb (70 kg) adult is 1.4 mg in a single sitting. — general population of the European Union, foods other than raw apricot kernels Evidence E source
  Acute reference dose 20 micrograms of cyanide per kg body weight; the opinion gives no separate figure for linseed
• The US National Center for Complementary and Integrative Health says there are theoretical reasons to suspect flaxseed or flaxseed oil might interact with anticoagulant or antiplatelet drugs, and advises anyone on medicines to ask their doctor first. — people taking anticoagulant or antiplatelet drugs Evidence E source
  Theoretical interaction with anticoagulant or antiplatelet drugs; advice to talk with a health care provider
• NCCIH says limited amounts of flaxseed or flaxseed oil may be safe for up to 6 months, that higher doses may cause bloating, fullness and diarrhea, and that raw or unripe seeds should not be eaten because they may contain potentially toxic compounds. — adults using flaxseed or flaxseed oil supplements Evidence E source
  Limited amounts may be safe short term (up to 6 months); higher doses may cause bloating, fullness and diarrhea; raw or unripe seeds may contain potentially toxic compounds

Who this works differently for

• In 72 children and adolescents with overweight, 20 g of flaxseed a day for four weeks eased mental fatigue but went with a smaller gain in height than puffed wheat (0.21 in / 0.53 cm against 0.43 in (1.09 cm), P = 0.03), and the authors advise against flaxseed during growth. — 72 healthy children and adolescents with BMI above 25, randomized to 20 g a day of flaxseed (n = 38) or 25 g a day of puffed wheat (n = 34) for 4 weeks (n = 72) Evidence B source
  Mental fatigue lower with flaxseed (P < 0.001 between groups); height gain 0.21 in (0.53 cm) (SD 0.89) against 0.43 in (1.09 cm) (SD 0.87), P = 0.03; BMI change -0.25 against -0.67, P = 0.01
• For ground linseed at the highest cyanide level in the EFSA database, 407 mg per kg, EFSA calculates that 1.3 to 14.7 g in one eating occasion reaches the acute limit of 20 mcg cyanide per kg body weight. At average body weight that is about 1.7 g for a toddler and 10.9 g for an adult. EFSA expects whole, unground seeds to give much lower exposure. — toddlers to adults, by body weight, EU back-calculation Evidence E source
  Ground linseed at 407 mg CN/kg reaches the ARfD at 1.3 to 14.7 g per eating occasion; at mean body weight 1.7 g for toddlers, 3.4 g for other children, 6.4 g for young adolescents, 9.0 g for adolescents, 10.9 g for adults
• EFSA notes that at the worst-case cyanide level a toddler would exceed the acute limit with about 4 g of ground linseed, roughly a teaspoon. It adds that a risk for adolescents cannot be excluded if linseed is ground, for example in a blender, and eaten at the 10 to 15 g three times a day that EMA recommends for constipation. — toddlers and adolescents, EU risk assessment Evidence E source
  Toddler exceeds the ARfD with about 4 g of ground linseed (a teaspoon) at the worst-case level; a risk for adolescents cannot be excluded at the EMA laxative dose of 10 to 15 g three times a day if the linseed is ground
• NCCIH says some studies suggest flaxseed is possibly unsafe in pregnancy, though the evidence is not conclusive, and that little is known about flaxseed while breastfeeding or about flaxseed oil in pregnancy or breastfeeding. — pregnant and breastfeeding women Evidence E source
  Flaxseed in pregnancy possibly unsafe, evidence not conclusive; safety of flaxseed in breastfeeding and of flaxseed oil in pregnancy or breastfeeding largely unknown

What is still unknown

• Flaxseed is not a substitute for fish oil: in the 13 randomized trials a scoping review could find, high-dose flaxseed oil raised the omega-3 index in nobody, and some studies recorded a fall, while microalgal oil raised it in every study. — 13 randomized controlled trials in vegetarians, vegans and other adults, January 2010 to September 2020, omega-3 index as the marker (n = 13 trials) Evidence B source
  No increase in the omega-3 index with high-dose flaxseed or echium oil, reductions in some studies; microalgal oil raised it in all studies
• EFSA reports that the food agency of Australia and New Zealand judged linseed foods not an appreciable risk, even though some exposure estimates passed the acute limit. Its reasons were that no human poisonings from linseed are reported and that volunteers who ate 3.5 oz (100 g) of ground linseed had no detectable blood cyanide. — Australia and New Zealand, FSANZ assessment as reported by EFSA Evidence E source
  High linseed bread intake gave estimates above the ARfD, yet no reported human poisonings from linseed; no blood cyanide after 100 g ground linseed in volunteers

The bottom line

Flaxseed does not stand in for fish oil, and this is the clearest thing on the page. A scoping review found 13 randomized trials measuring the omega-3 index. High-dose flaxseed oil raised that index in none of them, some trials recorded a fall, and microalgal oil raised it in every study. The review covers the oil rather than whole ground seed, and nothing in it was pooled. If you want EPA and DHA in your blood, the alpha-linolenic acid in flax is not the way there. In people who already have something wrong, the picture is partial. Sixteen randomized trials in 1,136 people with diabetes found weight, waist circumference and triglycerides down, while BMI, blood pressure, total cholesterol, LDL and HDL all stayed put, with heterogeneity of 85 to 96 percent on every outcome. Six small trials in 309 people with coronary artery disease found fasting glucose 8.35 mg/dL lower (95% CI 15.01 to 1.69, p = 0.01) and hs-CRP 1.35 mg/L lower (95% CI 1.93 to 0.77, p < 0.01), with weight and most lipids unchanged. The triglyceride result in that second analysis appeared only after a sensitivity analysis, which makes it the weakest number quoted here. Children are the caution with a trial behind it. Seventy-two children and adolescents with a BMI above 25 took either 20 g of flaxseed or 0.88 oz (25 g) of puffed wheat a day for four weeks. Mental fatigue eased on flaxseed (P < 0.001 between groups), and height gain over those four weeks was smaller than on puffed wheat, 0.21 in (0.53 cm) against 0.43 in (1.09 cm) (P = 0.03). Four weeks is too short to measure growth, so the height gap may be noise, and it is the only signal of its kind anywhere in this evidence. The authors still advise against flaxseed during growth, and we have no second trial to weigh against them. Two cautions are softer than they look. A review of 1,491 supplement and drug pairs put flaxseed among the three entities with the most documented contraindications, alongside echinacea and yohimbe, out of 152 contraindications counted in total. That is a count of what somebody wrote down rather than a measured risk, and it describes supplement doses. Flaxseed is also one of six edible seeds with documented hypersensitivity reactions, and the flax literature is case reports rather than series, so nobody can state a prevalence for flaxseed allergy. Reactions exist and can be severe, and no one has counted them. As for the cyanide in the seed, EFSA sets an acute reference dose of 20 micrograms per kilogram of body weight, which is 1.4 mg in one sitting for a 154 lb (70 kg) adult. For ground linseed at the highest cyanide level in its database, 407 mg per kg, EFSA calculates that about 10.9 g in one eating occasion reaches that limit for an average adult and about 1.7 g for a toddler. At that worst-case level a toddler passes the limit with about 4 g, roughly a teaspoon. EFSA adds that a risk for adolescents cannot be excluded at 10 to 15 g of ground linseed three times a day, and that whole seeds give much lower exposure. The same opinion weighs this against human data. In the study it relies on, 1.1 oz (30.9 g) of linseed carrying 6.8 mg of cyanide raised blood cyanide far less than cassava with the same amount, and EFSA judges its limit likely over-conservative for linseed. It also reports that the Australian and New Zealand food agency found no reported human poisonings from linseed. Since 2022 EU law caps hydrocyanic acid at 150 mg per kg in linseed sold to consumers, because ground linseed eaten raw releases more cyanide than whole or heat-treated seed. So the 30 g a day that carried the blood pressure effect sits above the worst-case adult figure for raw ground seed, while the one human study near that amount found a much smaller rise than the limit assumes. In a 30-day trial in 34 healthy adults, 0.35 to 1.4 oz (10 to 40 g) of milled flaxseed baked into a muffin caused no reported adverse events at any dose. NCCIH says limited amounts may be safe for up to 6 months, that higher doses may cause bloating, fullness and diarrhea, and that raw or unripe seeds should not be eaten. On medicines, NCCIH names anticoagulant and antiplatelet drugs as a theoretical interaction and advises anyone on medicines to ask their doctor first. No study on this page measured that interaction, and the contraindication count above still names no other drug. In pregnancy, NCCIH says some studies suggest flaxseed is possibly unsafe, though the evidence is not conclusive. Little is known about flaxseed in breastfeeding or about flaxseed oil in pregnancy or breastfeeding.

Sources

1. hf2-flxs-3 · EFSA scientific opinion
   In contrast, a lower bioavailability (peak level 6.5 μM after 60 min) was observed after ingestion of 30.9 g linseed also containing 6.8 mg cyanide. ... Linseed 6.40 ± 3.34 1.69–13.85 40 30–60 Apricot kernels 15.46 ± 5.12 7.48–22.59 20 5–40 Cassava 16.95 ± 5.96 10.31–31.87 30 22.5–52.5 ... The lower peak blood levels of cyanide observed after linseed as compared to cassava and apricot kernels containing equivalent amounts of bound cyanide can been explained by the lower activity of the degrading enzymes in linseed, in particular of the respective β‐glucosidase ... For exposure to cyanide from foods other than raw apricot kernels, bitter almonds and cassava roots, the release of cyanide is slower and the resultant blood levels are lower and the ARfD is likely to be over‐conservative.
   EFSA CONTAM Panel, Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels, EFSA Journal 2019;17(4):5662 · checked 2026-10-08
2. hf2-flxs-8 · randomized controlled trial
   Healthy, younger adults (n = 34; 18-49 years old) were randomized into four groups consuming one muffin daily for 30 days fortified with 10, 20, 30 or 40 g of milled flaxseed. ... 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.
   Eur J Nutr, 2016 · checked 2026-10-08
3. hf2-flxs-5 · EU regulation
   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. It is therefore appropriate to set stricter levels for whole linseed, which can be ground by the consumer, before consumption and ground linseed placed on the market for the final consumer when intended to be consumed raw. ... 8.3.2 Unprocessed whole, ground, milled, cracked, chopped linseed placed on the market for the final consumer ... 150 ... The maximum level does not apply to unprocessed whole, ground, milled, cracked, chopped linseed and unprocessed whole, ground, milled, cracked, chopped bitter almonds placed on the market for the final consumer in small quantities where the warning ‘Only to be used for cooking and baking. Do not consume raw!’ appears in the principal field of vision of the label
   Commission Regulation (EU) 2022/1364 of 4 August 2022, maximum levels of hydrocyanic acid in certain foodstuffs, OJ L 205/227 · checked 2026-10-08
4. flxs-r5-01 · meta-analysis of randomized controlled trials
   Meta-analysis of 33 trials (comprising 43 treatment arms) with 2427 participants revealed significant reductions in both systolic (WMD: -3.19 mmHg; 95% CI: -4.15 to -2.24, p < 0.001; I2 = 92.5%, p < 0.001) and diastolic blood pressure (WMD = -2.61 mmHg; 95% CI: -3.27, -1.94, p < 0.001; I2 = 94.1%, p < 0.001) following flaxseed supplementation.
   Food Funct, 2023 · checked 2026-09-24
5. flxs-r5-02 · meta-analysis of randomized controlled trials
   Flaxseed supplementation effectively reduces anthropometric indices and triglyceride levels in individuals with type 2 diabetes but does not suggest any benefit on blood pressure and other lipid profile parameters.
   Diabetes Metab Syndr, 2025 · checked 2026-09-24
6. flxs-r5-03 · meta-analysis of randomized controlled trials
   Flaxseed supplementation was associated with lowering FBS, hs-CRP, and TG levels but did not affect weight loss parameters and other lipid markers in CAD.
   Clin Cardiol, 2024 · checked 2026-09-24
7. flxs-r5-04 · meta-analysis of randomized controlled trials
   Overall, the flaxseed supplementation had a significant reduction in C-reactive protein (CRP) (SMD = -0.46; 95 % CI: -0.70, -0.23, P < 0.001; I2 = 82.9 %, P < 0.001), and interleukin 6 (IL-6) (SMD = -0.64, 95 % CI: -1.13, -0.16, P = 0.010; I2 = 92.7, P < 0.001).
   Prostaglandins Other Lipid Mediat, 2024 · checked 2026-09-24
8. flxs-r5-07 · systematic review of case documentation
   Flaxseed, echinacea, and yohimbe had the largest number of documented contraindications.
   Int J Clin Pract, 2012 · checked 2026-09-24
9. flxs-r5-08 · review of case reports
   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.
   Allergy, 2016 · checked 2026-09-24
10. flxs-r5-09 · agency scientific opinion
    an acute reference dose (ARfD) of 20 μg/kg body weight (bw) was established for cyanide (CN)
    EFSA Journal, 2019, opinion 5662, cyanogenic glycosides in foods other than raw apricot kernels · checked 2026-09-24
11. hf2-flxs-6 · agency advice
    There are theoretical reasons to suspect that flaxseed or flaxseed oil might interact with other drugs, such as anticoagulant or antiplatelet drugs. If you take any type of medicine, talk with your health care provider before using flaxseed, flaxseed oil, or other herbal products; some herbs and medicines interact in harmful ways.
    National Center for Complementary and Integrative Health (NIH), Flaxseed and Flaxseed Oil, What Do We Know About Safety? · checked 2026-10-08
12. hf2-flxs-9 · agency advice
    Don’t eat raw or unripe flaxseeds; they may contain potentially toxic compounds. Limited amounts of flaxseed (especially flaxseed lignan extract or mucilage) or oral flaxseed oil supplements may be safe for use in the short-term (up to 6 months). Higher doses of flaxseed or flaxseed oil supplements may cause uncomfortable digestive symptoms like bloating, fullness, and diarrhea.
    National Center for Complementary and Integrative Health (NIH), Flaxseed and Flaxseed Oil, What Do We Know About Safety? · checked 2026-10-08
13. flxs-r5-06 · randomized controlled trial
    Flaxseed showed less benefit than puffed wheat on anthropometric measures as it caused a smaller increase in height (0·53 (sd 0·89) v. 1·09 (sd 0·87) cm, P = 0·03)
    Br J Nutr, 2021 · checked 2026-09-24
14. hf2-flxs-1 · EFSA scientific opinion
    Table 23 shows the estimated maximum amount of ground linseed that can be consumed without exceeding the ARfD of 20 μg/kg bw using the highest value reported in the EFSA database (407 mg CN/kg) as a worst‐case approach. Depending on the body weight of the individual, consumption of 1.3–14.7 g of ground, linseed containing CN at this level, reaches the ARfD ... It can be expected that consumption of intact linseed (i.e. not freshly ground) would lead to much lower cyanide exposures. ... Toddlers 8.7 11.9 15.9 407 a 3 0.02 1.3 1.7 2.3 Other children 14 23.1 37 407 a 3 0.02 2.1 3.4 5.5 Young adolescents 29.4 43.4 62 407 a 3 0.02 4.3 6.4 9.4 Adolescents 45 61.3 83 407 a 3 0.02 6.6 9.0 12.2 Adults 52 73.9 100 407 a 3 0.02 7.7 10.9 14.7
    EFSA CONTAM Panel, Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels, EFSA Journal 2019;17(4):5662 · checked 2026-10-08
15. hf2-flxs-2 · EFSA scientific opinion
    ... recommends consumption of 10–15 g of linseed (whole or ‘broken’ linseed) three times a day to treat constipation in adults and adolescents aged over 12 years. For ground linseed containing the highest level of measured total CN levels (407 mg/kg) as a worst‐case scenario, the ARfD would be exceeded by a toddler when consuming about 4 g of ground linseed (roughly a tea spoon). Taking into account all uncertainties, a risk for adolescents cannot be excluded if ground linseed (e.g. when put in a blender) is consumed at the amount recommended by the EMA.
    EFSA CONTAM Panel, Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels, EFSA Journal 2019;17(4):5662 · checked 2026-10-08
16. hf2-flxs-7 · agency advice
    Some studies of the use of flaxseed during pregnancy suggest it is possibly unsafe, but the evidence is not conclusive. Little is known about whether it is safe to use flaxseed while breastfeeding. Little is known about whether it is safe to use flaxseed oil during pregnancy or while breastfeeding.
    National Center for Complementary and Integrative Health (NIH), Flaxseed and Flaxseed Oil, What Do We Know About Safety? · checked 2026-10-08
17. flxs-r5-05 · scoping review of randomized trials
    High dose flaxseed or echium seed oil supplements, provided no increases to O3I and some studies showed reductions. However, microalgal oil supplementation increased O3I levels for all studies.
    Crit Rev Food Sci Nutr, 2022 · checked 2026-09-24
18. hf2-flxs-4 · EFSA scientific opinion
    Although acute exposures with linseed containing bread exceeded the ARfD, FSANZ concluded that linseed and foods containing linseed do not represent an appreciable health risk as there are not reports in the literature of human poisonings upon consumption of linseed and in a study in which human volunteers consumed 100 g of ground linseed no cyanide was detected in the blood
    EFSA CONTAM Panel, Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels, EFSA Journal 2019;17(4):5662 · checked 2026-10-08