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Other common name(s): flaxseed oil, linseed, lint bells, linum

Scientific/medical name(s): Linum usitatissimum


Flax is an annual plant cultivated for its fiber, which is used in making linen. Flaxseed and its oil are used in herbal remedies.


Flaxseed and its oil have been promoted since the 1950s as a food and dietary supplement with anti-cancer properties. Most of the evidence of its ability to prevent the growth or spread of cancer has come from studies in animals. Only recently has there been some clinical evidence suggesting that flaxseed supplements may be useful in men with early-stage prostate cancer, but not all the studies have found this. Controlled clinical studies are needed to determine its usefulness in preventing or treating prostate cancer and other forms of cancer in humans. It is important to know which studies were looking at whole flaxseed or flaxseed oil, since the lignans and fiber are only available in the flaxseeds and flaxseed meal, not the oil.

How is it promoted for use?

Herbalists promote the use of flaxseed for constipation, abdominal problems, breathing problems, sore throat, eczema, menstrual problems, and arthritis.

The oil extracted from flaxseeds is said to lower cholesterol levels, boost the immune system, and prevent cancer. Flaxseed oil is high in alpha-linolenic acid, an omega-3 fatty acid that is thought to have beneficial effects against cancer when consumed (see our document called Omega-3 Fatty Acids). Omega-3 fatty acids can also be found fish and fish oils, as well as in some other plant oils.

Recently, attention has focused on the flaxseed itself, which is a rich source of lignans, compounds that can act as anti-estrogens or as weak estrogens. It is thought that lignans may play a role in preventing estrogen-dependent cancers, such as breast cancer and other types of cancer. Lignans may also function as antioxidants and, though mechanisms that are not yet fully understood, may slow cell growth. When flaxseed is eaten, the lignans are activated by bacteria in the intestine. Lignans are not present in the flaxseed oil.

What does it involve?

Flaxseed is available in the form of flour, meal, and seeds. It may be found in some multigrain breads, cereals, breakfast bars, and muffins. The toasted seeds are sometimes mixed into bread dough or sprinkled over salads, yogurt, or cereal. Flaxseed meal can be used in the same ways. Flaxseed oil is available in many health food stores in liquid form and is sometimes mixed into cottage cheese or other foods. The oil is also available in softgel capsules.

What is the history behind it?

Flaxseed was cultivated by the Babylonians in 3000 BC. In the 1950s, a German biochemist named Johanna Budwig first promoted flaxseed oil as a treatment through a diet she devised for cancer patients. The diet was a strict regimen that avoided sugar, animal fats, salad oil, meats, butter, and margarine. The patients were given flaxseed oil mixed with cottage cheese and milk, and meals high in fruits, vegetables, and fiber. She claimed that within 3 months, some patients on this diet had smaller tumors, some had no tumors left, and all felt better.

What is the evidence?

Most of the evidence for an anti-cancer effect of flaxseed and flaxseed oil comes from research using laboratory animals or cells grown in laboratory dishes. In one cell culture study, flaxseed lignans reduced stickiness and movement of breast cancer cells, both properties related to the cancer's ability to spread or metastasize. Researchers have also found that mice fed flaxseed had reduced formation, growth, or spread of prostate cancer, breast cancer, and melanoma. Flaxseed reduced pre-cancerous colon polyps in one study of rats, but in another study showed no effect on intestinal cancer in mice. In a 2007 report, flaxseed reduced growth of breast cancer cells in mice and enhanced the effectiveness of tamoxifen, a standard drug for hormonal therapy. Since then, other studies have looked at tamoxifen and flaxseed in animals, but human studies are still needed to find out if the effects hold true for humans.

There have been some small studies of the effects of flaxseed in humans. A small study of 15 men found that a low-fat diet supplemented with flaxseed lowered their blood prostate-specific antigen (PSA) levels and slowed the growth of benign prostate cells. Another study of 25 men with prostate cancer found that a low-fat diet along with ground flaxseed reduced serum testosterone, slowed the growth rate of cancer cells, and increased the death rate of cancer cells. A 2008 study looked at men with prostate cancer who were scheduled for surgery. Researchers gave some men flaxseed while others ate their usual diets; some were put on low fat diets. The men who got the flaxseed had slower cancer growth than those who did not, regardless of total fat intake. This was a fairly small study that did not look at survival or relapse rates. A 2011 study that compared flaxseed and low fat diets in men with prostate cancer found that a low fat diet, but not flaxseed, helped lower certain factors in the blood that are linked to prostate cancer growth.

More research in humans is needed to determine the usefulness of flaxseed in cancer treatment and prevention. Animal and laboratory studies show promise, but human studies are conflicting. Further studies are needed to be sure which results apply to humans.

Are there any possible problems or complications?

This product is sold as a dietary supplement in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

The immature pods of flaxseed are poisonous and should never be used. It is important for those taking flax seed oil to be sure that it is labeled for human consumption.

Some possible side effects include diarrhea, gas, and nausea. Flaxseed oil should not be used with other laxatives or stool softeners. People who have inflammatory bowel disease or narrowing of the intestine, esophagus, or stomach should avoid flaxseed. Flaxseed is also not recommended by some doctors for people with diabetes.

Some medicines and supplements may not be absorbed properly if they are taken at the same time as flaxseed. For this reason, some doctors recommend taking medicines 1 or 2 hours before or after taking flaxseed.

Flaxseed may also interfere with x-rays taken after a barium enema, and some doctors recommend avoiding taking flaxseed before undergoing this test. In addition, the potential interactions between flaxseed and other drugs and herbs should be considered. Some of these combinations may be dangerous. Always tell your doctor and pharmacist about any herbs you are taking.

A few severe allergic reactions to flaxseed have been reported. Those allergic to other plants may be more likely to be allergic to flax.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

Flaxseeds and flaxseed oil can spoil if they are not kept refrigerated. They should be protected from light, heat, air, and moisture.

To learn more

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-227-2345).

Dietary Supplements: What Is Safe?

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods and Cancer

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer


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Bergman Jungestrom M, Thompson LU, Dabrosin C. Flaxseed and its lignans inhibit estradiol-induced growth, angiogenesis, and secretion of vascular endothelial growth factor in human breast cancer xenografts in vivo. Clin Cancer Res. 2007;13:1061-1067.

Chen J, Hui E, Ip T, Thompson LU. Dietary flaxseed enhances the inhibitory effect of tamoxifen on the growth of estrogen-dependent human breast cancer (mcf-7) in nude mice. Clin Cancer Res. 2004;10:7703-7711.

Chen J, Power KA, Mann J, Cheng A, Thompson LU. Flaxseed alone or in combination with tamoxifen inhibits MCF-7 breast tumor growth in ovariectomized athymic mice with high circulating levels of estrogen. Exp Biol Med. 2007;232:1071-1080.

Chen J, Stavro PM, Thompson LU. Dietary flaxseed inhibits human breast cancer growth and metastasis and downregulates expression of insulin-like growth factor and epidermal growth factor receptor. Nutr Cancer. 2002;43:187-192.

Chen J, Wang L, Thompson LU. Flaxseed and its components reduce metastasis after surgical excision of solid human breast tumor in nude mice. Cancer Lett. 2006;234:168-175.

Demark-Wahnefried W, Polascik TJ, George SL, et al. Flaxseed supplementation (not dietary fat restriction) reduces prostate cancer proliferation rates in men presurgery. Cancer Epidemiol Biomarkers Prev. 2008 Dec;17(12):3577-3587.

Demark-Wahnefried W, Price DT, Polascik TJ, Robertson CN, Anderson EE, Paulson DF, Walther PJ, Gannon M, Vollmer RT. Pilot study of dietary fat restriction and flaxseed supplementation in men with prostate cancer before surgery: exploring the effects on hormonal levels, prostate-specific antigen, and histopathologic features. Urology. 2001;58:47-52.

Demark-Wahnefried W, Robertson CN, Walther PJ, Polascik TJ, Paulson DF, Vollmer RT. Pilot study to explore effects of low-fat, flaxseed-supplemented diet on proliferation of benign prostatic epithelium and prostate-specific antigen. Urology. 2004;63:900-904.

Haggans CJ, Hutchins AM, Olson BA, Thomas W, Martini MC, Slavin JL. Effect of flaxseed consumption on urinary estrogen metabolites in postmenopausal women. Nutr Cancer. 1999;33:188-195.

Heymach JV, Shackleford TJ, Tran HT, et al. Effect of low-fat diet on plasma levels of NF-{kappa}B-regulated inflammatory cytokines and angiogenic factors in men with prostate cancer. Cancer Prev Res (Phila). 2011 Jul 15.

Lin X, Gingrich JR, Bao W, Li J, Haroon ZA, Demark-Wahnefried W. Effect of flaxseed supplementation on prostatic carcinoma in transgenic mice. Urology. 2002;60:919-924.

Memorial Sloan-Kettering Cancer Center Web site. Flaxseed. http://www.mskcc.org/mskcc/html/69220.cfm. Accessed July 20, 2011.

Natural Standard. Herbal/plant therapies: flaxseed and flaxseed oil (linum usitatissimum). Complementary/Integrative Medicine Education Resources, The University of Texas M.D. Anderson Cancer Center Web site. Accessed at www.mdanderson.org/departments/cimer/display.cfm?id=0D86E843-44DF-4766-9D743BA1FE8B9877&method=displayFull on June 4, 2008. Content no longer available.

Yan L, Yee JA, Li D, McGuire MH, Thompson LU. Dietary flaxseed supplementation and experimental metastasis of melanoma cells in mice. Cancer Lett. 1998;124:181-186.

Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 10/14/2011
Last Revised: 10/14/2011