Flaxseed

Last reviewed October 2026 · Based on 10 studies

Bottom line

Pooled trials find ground flaxseed modestly lowers blood pressure and LDL, total cholesterol and triglycerides, with the clearest results at around 30 g a day for at least 12 weeks. It does not raise HDL cholesterol. Its fiber can cause gut symptoms, and it may interact with blood-thinning medicines.

What it helps with

Lowers blood pressure — Benefit, evidence grade C (Limited). Meta-analyses of RCTs with consistent direction; downgraded for very high heterogeneity and because blood pressure is a surrogate marker.
Lowers LDL cholesterol — Benefit, evidence grade B (Moderate). Meta-analyses of many RCTs with consistent direction; downgraded once because LDL is a surrogate marker.
Lowers total cholesterol — Benefit, evidence grade B (Moderate). Meta-analysis of 62 RCTs with 3772 participants; downgraded once because total cholesterol is a surrogate marker.
Lowers triglycerides — Benefit, evidence grade B (Moderate). Meta-analysis of 62 RCTs plus a supporting trial; downgraded once because triglycerides are a surrogate marker.
Raises HDL cholesterol — No effect, evidence grade B (Moderate). Two meta-analyses of RCTs consistently found no HDL change; downgraded once because HDL is a surrogate marker.

What it is

Flaxseed (also called linseed) is the seed of the flax plant, eaten whole, ground, or as oil [1]. It is rich in the plant omega-3 fat alpha-linolenic acid (ALA), lignans and dietary fiber [1]. Ground seed gives high bioavailability of ALA and lignans, while whole seeds pass through less digested, so ground form is the most useful way to eat it [1].

How it might work

The proposed mechanisms involve its ALA, lignans and soluble fiber acting through anti-inflammatory and antioxidant effects, better lipid and glucose handling, gut microbiota changes and healthier blood vessel function [2]. These pathways are proposed, not directly shown, to lower cardiometabolic risk [2].

What the research says

Blood pressure: a 2023 dose-response review of 33 trials with 2,427 people found systolic pressure down 3.19 mmHg and diastolic down 2.61 mmHg [3]. Two earlier reviews agree, with systolic down 2.85 and diastolic down 2.39 mmHg in 15 trials with 1,302 people [4], and systolic down 1.77 and diastolic down 1.58 mmHg in 11 studies [5]. The largest drops came with at least 30 g a day, use beyond 12 to 20 weeks, whole ground seed, and participants with hypertension or overweight [3][5]. A 2022 trial in 112 hypertensive patients found 30 g a day for 12 weeks cut systolic by 13.38 points versus a 1.72-point rise with placebo, and diastolic by 5.6 points versus a 2.39-point rise [6].

Cholesterol and triglycerides: a 2020 review of 62 trials with 3,772 people found total cholesterol down 5.39 mg/dL, triglycerides down 9.42 mg/dL and LDL down 4.21 mg/dL, with HDL unchanged [7]. A 2022 review of 14 trials with 1,107 people found the same pattern in dyslipidemic patients, with larger LDL gains when BMI exceeded 25 [8]. In 64 ulcerative colitis patients, 30 g a day of ground flaxseed for 12 weeks lowered total cholesterol and triglycerides and raised HDL [9].

Not all groups respond: a 2025 review of 16 trials with 1,136 diabetic patients found lower triglycerides but no significant change in blood pressure, total cholesterol, LDL or HDL [10]. Reviews report very high variation between trials, and blood pressure and cholesterol are risk markers rather than heart attacks or strokes [3][7].

Doses used in studies

Trials typically used 10 to 30 g a day of ground flaxseed or its oil for 8 to 12 weeks or longer [6][9]. The clearest blood pressure and lipid results came with around 30 g a day of ground whole seed for at least 12 weeks, and pressure effects grew further beyond 20 weeks [3][5]. Reviews suggest starting with 1 to 2 tablespoons a day and increasing as tolerated [2]. These are study doses, not recommendations.

Safety and interactions

Flaxseed is generally considered safe in moderate amounts [2]. Common gut symptoms include bloating, gas, stomach pain and diarrhea from its high fiber content, worse at higher doses [2]. Its ALA may reduce platelet aggregation and interfere with blood clotting, especially with anticoagulant medicines, so people on blood thinners should ask a clinician first [2]. It contains small amounts of cyanogenic glycosides, generally safe and reduced by processing, though safety of chronic high-dose use is understudied [2]. Its lignans have mild estrogen-like effects, so people with hormone-sensitive conditions should be cautious, and pregnant or breastfeeding women and people on hormone therapies should ask a clinician before large amounts [2]. No upper intake limit or toxic dose was identified in these sources, and the trials reported no data on interactions with blood pressure medicines beyond the general caution to check with a clinician [2][1].

Caveats and open questions

Trial results vary widely, with heterogeneity above 90% in the largest blood pressure review, and most trials were short, limiting conclusions about long-term effects [3][2]. Diabetic patients saw triglyceride but not blood pressure or other lipid gains, and one colitis trial saw no blood pressure change, so benefits are not uniform [10][9]. Direct evidence linking flaxseed to fewer cases of hypertension, diabetes, heart disease or longer life is limited [2].