Flaxseed, rich in omega-3 fatty acids and phytoestrogenic lignans, has been used in traditional medicine to treat coughs, colds, constipation, urinary tract infections, and as a topical demulcent and 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 to reduce cyclical mastalgia ⁽³⁹⁾.

In other studies, supplementation with a major lignan derived from flaxseed improved glycemic control in Type-2 diabetic patients ⁽¹³⁾. However, it was ineffective in reducing hot flashes in postmenopausal women with or without breast cancer ⁽²⁸⁾. Flaxseed products have also been shown to reduce blood pressure ^{(1) (34) (40) (41)}. In pre-diabetic patients, daily intake of flaxseed powder lowered blood pressure but did not improve insulin resistance or glycemic indices ⁽³⁵⁾. Supplementation along with lifestyle modification was more effective than lifestyle modification alone to manage metabolic syndrome ⁽³⁶⁾ and non-alcoholic fatty liver disease ⁽³⁷⁾. Flaxseed was also found more effective than psyllium for improving symptoms of constipation, as well as glycemic and lipid levels ⁽⁴²⁾. In patients with ulcerative colitis, both flaxseed and flaxseed oil were reported to reduce inflammatory markers, disease severity, and blood pressure ⁽⁴⁴⁾. Daily flaxseed oil consumption also significantly reduced inflammatory markers related to cardiovascular disease ⁽³⁸⁾, and may help reduce bone resorption ⁽⁴³⁾ in hemodialysis patients. Furthermore, flaxseed conferred renoprotection in patients with lupus nephritis ⁽³⁾, but additional studies are needed.

Although flaxseed-derived lignan reduced blood glucose levels in hypercholesterolemic individuals ⁽¹⁴⁾, data on its cholesterol-lowering effects are mixed ^{(11) (15) (26)}.

Flaxseed has also been investigated for its anticancer potential. Preclinical studies suggest it may inhibit growth and metastasis of human breast ^(6) (29), prostate ⁽⁷⁾ cancers and melanoma ⁽⁸⁾; and may reduce radiation therapy-induced lung damage and improve survival ⁽²⁷⁾. Flaxseed was reported to lower tumor biomarkers in patients with prostate ^(9) (25) and breast cancer ⁽¹⁰⁾. But 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 the 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.