The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.


Avoid with known allergy or sensitivity to flaxseed, flaxseed oil, its parts, or any other members of the Linaceae plant family.

Allergic reactions to flaxseed and flaxseed powder have been reported. Itchy palms, hives, itchy eyes and weeping, nausea, and vomiting after consuming flaxseed oil was reported in one case. In another case, a man experienced 5-6 episodes of intestinal and stomach pain, vomiting, diarrhea, hives, shortness of breath, watery discharge, sneezing, stuffy nose, itching, and generally felt unwell after eating bread containing flaxseed.

Anaphylaxis (severe allergic reaction) has also been reported.

Side Effects and Warnings

Both flaxseed and flaxseed oil are likely safe when taken by mouth in suggested doses for under four months by healthy people. A part of flaxseed, alpha-linolenic acid (ALA), has been well tolerated for up to five years as part of the Mediterranean diet. Flaxseed should be ingested with adequate fluid intake (1:10 seed: liquid is suggested). Application to the skin of the seed form or poultice is generally well tolerated.

Flaxseed or flaxseed oil is possibly safe when used for more than four months. Flaxseed or flaxseed oil is possibly safe when used in pregnant or lactating women in amounts normally consumed in food or under guidance of a practitioner.

Flaxseed may lower blood sugar levels. Caution is advised in people with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Blood glucose levels may need to be monitored by a qualified healthcare professional, including a pharmacist, and medication adjustments may be necessary.

Flaxseed may increase the risk of bleeding. Caution is advised in people with bleeding disorders or taking drugs that may increase the risk of bleeding. Dosing adjustments may be necessary.

Flaxseed may cause low blood pressure. Caution is advised in people taking drugs or herbs and supplements that lower blood pressure.

Use cautiously in people with prostate cancer, high triglycerides, or hypothyroidism (underactive thyroid).

Use cautiously in women with hormone-sensitive conditions or women taking estrogens. Use cautiously pregnant or lactating women. Use cautiously in people taking laxatives, furosemide, or ketoprofen.

Avoid in people with known allergy or sensitivity to flaxseed, flaxseed oil, its constituents, or any other members of the Linaceae plant family.

Avoid in people with acute or chronic diarrhea, diverticulitis (colon disorder), or inflammatory bowel disease. Avoid use in open wounds or scraped surfaces. Avoid flaxseed (not flaxseed oil) in people with narrowing of the esophagus/stomach/intestines or bowel obstruction.

Intestinal obstruction may occur when large amounts of flaxseed are ingested, or when taken without enough water or liquid (1:10 seed: liquid is suggested). Immature flaxseed seedpods may be poisonous and should not be consumed.

Flax may also cause abdominal pain and bloating, altered estrogen activity, bowel obstruction, cell damage, change in bowel habits, decreased absorption of drugs, vitamins or minerals, diarrhea, eye itching and weeping, gas, headache, higher or lower cholesterol, hives, increase in total red blood cell count, increased bleeding time, increased blood cyanide levels, increased prostate cancer risk, indigestion, itching, loose stools, malaise (feeling unwell), mania or hypomania, nausea, paralysis, prolonged luteal phase, rapid breathing, seizures, shortness of breath, sneezing, unstable gait, vomiting, watery discharge, weakness, weight gain or loss.

Pregnancy and Breastfeeding

Human evidence of medicinal use of flaxseed or flaxseed oil during pregnancy and lactation is limited.

A study reported that flaxseed oil supplementation led to a significant increase in ALA, eicosapentaenoic acid (EPA), and docosapentaenoic acid (DPA) in the breast milk. Fatty acid levels in breast milk returned to baseline one week after discontinuing supplementation.

In a survey, flax was reported to be one of the most frequently used herbal products by the pregnant women surveyed.

This evidence-based monograph was prepared by The Natural Standard Research Collaboration