Flaxseed, rich in omega-3 fatty acids and phytoestrogenic lignans, is used in traditional medicine to treat coughs, colds, constipation, urinary tract infections, and as a topical demulcent and an emollient. In clinical studies, flaxseed improved mild menopausal symptoms ⁽¹¹⁾; and exerted chemoprotective effects ⁽²⁾ along with affecting moderate reductions in estrogens and androgens ⁽²⁴⁾ in postmenopausal women. It may also benefit women with polycystic ovarian syndrome by reducing androgen levels ⁽²³⁾; and flaxseed powder combined with Vitex agnus may help reduce cyclical mastalgia ⁽³⁹⁾. Supplementation with a major lignan derived from flaxseed improved glycemic control in Type-2 diabetic patients ⁽¹³⁾. But it was ineffective in reducing hot flashes in postmenopausal women with or without breast cancer ⁽²⁸⁾.
In other studies, flaxseed products were found to reduce blood pressure ^{(1) (34) (40) (41) (35) (53)} but did not improve insulin resistance or glycemic indices in pre-diabetic patients ⁽³⁵⁾ although conflicting data suggest otherwise ^(49) (54). Supplementation along with lifestyle modification was more effective than lifestyle modification alone to manage metabolic syndrome ⁽³⁶⁾ and non-alcoholic fatty liver disease ^(37) (46). Systematic reviews determined that flaxseed may have a role in decreasing appetite and hunger ⁽⁵⁵⁾, improving obesity ⁽⁵⁸⁾, preventing cardiovascular events by inhibiting platelet aggregation ⁽⁵⁶⁾ and have beneficial effects on apolipoproteins ⁽⁵⁹⁾.  A plant-based diet that included flaxseed was associated with reduced risk of cardiovascular disease and stroke ⁽⁶⁰⁾. Flaxseed-derived lignan also decreased blood glucose levels in hypercholesterolemic individuals ⁽¹⁴⁾, but data on its cholesterol-lowering effects are mixed ^{(11) (15) (26) (61)}.

Flaxseed was also found more effective than lactulose ⁽⁴⁷⁾ and psyllium for improving symptoms of constipation ⁽⁴²⁾. In patients with ulcerative colitis, both flaxseed and flaxseed oil may affect reductions in systemic inflammation ⁽⁵⁰⁾ as well as disease severity ⁽⁴⁴⁾. In addition, daily flaxseed oil consumption significantly decreased inflammatory markers related to cardiovascular disease ⁽³⁸⁾, and may help reduce bone resorption ⁽⁴³⁾ in hemodialysis patients. Flaxseed was also shown to confer renoprotection in patients with lupus nephritis ⁽³⁾; and may benefit patients with rheumatoid arthritis ⁽⁵¹⁾.

Preclinical studies suggest that flaxseed may inhibit growth and metastasis of human breast ^(6) (29), prostate ⁽⁷⁾ cancers and melanoma ⁽⁸⁾; and reduce radiation therapy-induced lung damage and improve survival ⁽²⁷⁾. It was also shown to lower tumor biomarkers in patients with prostate ^(9) (25) and breast cancer ⁽¹⁰⁾; and may confer protection against radiation-induced adverse effects in lung cancer patients although such effects could not be accurately determined due to poor tolerability of flaxseed ⁽⁵²⁾. In other studies, a flaxseed extract was ineffective in preventing oral infection following radiation treatment in head and neck cancer patients ⁽¹²⁾; and supplementation with secoisolariciresinol diglucoside, a lignan, was not superior to placebo in reducing proliferation in benign breast tissue of premenopausal women at increased risk of breast cancer ⁽⁴⁵⁾.

Flaxseed ingestion can increase urinary lignan excretion ⁽⁴⁾ and the length of luteal phase of the menstrual cycle ⁽⁵⁾. Because flaxseed has phytoestrogenic effects, patients with estrogen receptor positive (ER+) breast cancer should consult their physicians before using it.