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Flaxseed side-effects, risks and warnings - Nutrition Myths
Flaxseed side-effects


  • Due to insufficient studies on long-term effects, flaxseeds and flaxseed oil have not been included in GRAS “safe food foods” list.
  • Studies so far show that consumption is generally safe for healthy individuals. However, the results are either conflicting or unclear.
  • Flaxseeds contain anti-nutrients, which are of concern in the unripe seeds.
  • High fiber in flaxseeds may reduce the absorption of some nutrients and medications.
  • Flaxseeds may cause gastrointestinal problems, when unaccustomed to a high fiber intake.
  • Flaxseeds are unsafe for those who suffer from allergy to Linaceae family plants and who are sensitive to flaxseed oil.
  • Flaxseeds are possibly unsafe during pregnancy and in hormone-related cancers.


Flaxseed side-effects, risks and warnings

Flaxseeds are mostly known for their dense nutrient composition and health benefits similar to chia seeds. Flaxseed side-effects are, however, not uncommon and worth considering before adding them to our diet. 

Although flaxseed has not been added to the “generally regarded as safe” (GRAS) list of the FDA (1), available studies show that both flaxseed oil and whole flaxseed consumption are safe for healthy individuals. Additionally, the FDA allows 12% of flaxseeds to be added to foods. (2, 3)

Please note that no adverse effects have been observed in people that consumed 50g of flaxseeds per day. (4)

This article lists all the known concerns about flaxseed side-effects and contraindications. (5)

The following points refer to whole or milled flaxseeds, unless when specifically referring to extracts, such as lignans or oil.

Flaxseed side-effects related to anti-nutrients

Anti-nutrients are substances that interfere with the digestion and absorption of nutrients. They can be naturally occurring or synthetic.

There are two major anti-nutrients in flaxseeds, cyanogenic glycosides and phytic acid.

Cyanogenic glycosides

Cyanogenic glycosides are the major anti-nutrient in flaxseeds.

This compound releases hydrogen cyanide in the intestine that interferes with bodily functions and organs (e.g. it is a potent respiratory inhibitor and affects the intake of iodine by the thyroid gland).

When flaxseeds are included in baked products (up to 28%), the released cyanide due to the heating process is below harmful limits to humans.

However, when flaxseeds are used as an oil extract, the remaining flaxseed meal used for animal feed requires detoxification of cyanide by solvent extraction. (6)

Immature flaxseeds contain much higher amounts of this compound, so they can be poisonous. For this reason, they should not be eaten. (7)

Buy flaxseeds from a known and trusted supplier, to ensure that they are mature.

Phytic acid

Phytic acid reduces the absorption of minerals, such as calcium, zinc, magnesium, copper and iron. It binds these minerals with proteins, making them less bioavailable.


  • Immature flaxseeds should not be eaten, due to the high contents of toxic cyaadnogenic glycosides
  • Phytic acid reduces the absorption of some minerals
  • Anti-nutrients in flaxseeds do not pose health risks in the amounts that flaxseeds are commonly consumed

Flaxseed side-effects related to gastrointestinal problems

Since they are rich in fiber, flaxseeds generally improve bowel movements. However, they may also cause gastrointestinal side-effects, such as constipation, diarrhea or bloating.

These negative effects usually occur with a sudden increase of fiber intake above the usual dosage. Therefore, it is best to introduce high fiber foods or supplements gradually.

An increase in fiber intake must also be accompanied with an increase in water intake to prevent the bulk-forming related blockages.

The water intake must be increased in the proportion of one part of flaxseed to 10 parts of water.

Taking laxatives and stools softeners together with flaxseeds (but not flaxseed oil), may add to the laxative effects.  


  • Flaxseeds are high in fiber and may cause some gastrointestinal effects
  • To avoid unpleasant gastrointestinal reactions, increase the amounts of flaxseeds gradually
  • Drink plenty of water while increasing the fiber intake
  • Flaxseeds may add to the laxative effects, when used at the same time with stool softener medications or natural remedies.

Flaxseed side-effects related to hormones

Flaxseeds are rich in phytoestrogens called, lignans. They have unclear effects on medical conditions related to hormones, such as endometriosis and breast, uterine and prostate cancers. (8)

It is still unclear how lignans interact with oral contraceptives and hormone replacement therapy.  However, it is known that they have a significant impact on the endocrine system related to estrogen.

The evidence of these side-effects is still weak. More studies are needed, but precautions should be taken.

It is currently considered as “possibly unsafe” in hormone related conditions.


  • Lignans in flaxseeds may have an effect on hormone sensitive conditions.
  • People with risk factors for these conditions should avoid flaxseeds and flaxseed oil.

Flaxseed side-effects in pregnancy

While there is not enough scientific data on the effects of flaxseeds during pregnancy in humans, animal studies show possible harmful effects related to hormonal changes.

For this reason, flaxseed or flaxseed oil consumption during pregnancy is not advisable and should be avoided.


  • Flaxseeds and flaxseed oil are possibly unsafe for pregnant women

Flaxseed side-effects related to prostate cancer

Studies associating Alpha-linolenic acids with prostate cancer show conflicting results. Some found evidence of an increased cancer risk, while others show protective properties.

More studies are needed. In the meantime, flaxseeds are not advised for men with prostate cancer or at risk of developing prostate cancer. Overall, flaxseeds should not be consumed in large amounts by men.


  • High flaxseed intake, due to ALA contents, may be associated with prostate cancer risk.
  • Although more studies are needed, men should avoid consuming large amounts of flaxseeds.

Flaxseeds side-effects related to type 2 diabetes

Fiber in flaxseeds can slow down the release of sugars from the stomach and lower blood sugar levels. (9, 10)

The effects of anti-diabetic drugs may be potentially increased by flaxseeds, resulting in an extremely low blood sugar level.

See your doctor, if you are on anti-diabetic drugs, before the introduction of flaxseeds in your diet.


  • When on blood sugar regulating medications, flaxseeds may enhance the blood sugar lowering effect.

Allergic reaction to flaxseeds

Although not very common, allergy to flaxseed or flaxseed oil can occur.

Flaxseeds contain potent allergens that can affect some people, which may even result in anaphylaxis. (11)

It is thought that proteins, such as oleosins (structural proteins), are the main allergens. (12)

Flaxseeds and flaxseeds oil should be avoided, if you are allergic to plants of the Linaceae family.


  • Although rare, allergic reactions (including anaphylaxis) to flaxseeds may occur.

Hypersensitivity to flaxseed components

Hypersensitivity has been reported, due to occupational exposure to dust from processed flaxseeds. These reactions are anecdotal and haven’t been shown in a clinical environment.

Other sensitivities involve flaxseed oil consumption with symptoms such as palmar pruritis, abdominal pain, vomiting or nasal obstruction. (8)


  • Non-allergic sensitivity may occur, when workers are constantly exposed to flaxseed dust
  • Hypersensitivity reactions can occur from flaxseed oil consumption.

Flaxseed side-effects related to blood clotting

Flaxseeds and flaxseed oil decrease platelet aggregation and may increase the risk of bleeding. (8)

Although not yet proven by clinical trials, the effects of anticoagulants, antiplatelet agents and nonsteroidal anti-inflammatory drugs (NSAIDs) may be potentiated by flaxseeds, which may result in increased bruising and bleeding.


  • Avoid flaxseeds and flaxseed oil when taking anticoagulants, antiplatelet agents, NSAIDs and anticoagulant herb supplements.

Flaxseed and risk of blockage

Flaxseeds can absorb liquids between 16-30 times its dry weight, thanks to high amounts of polysaccharides in the seed coating (7).

Eating large quantities of flaxseeds in one meal, may pose a danger of the flax fiber obstructing the air passage.

While this applies to everybody, special care must be taken by people at risk of obstruction (e.g. with history of esophageal, gastrointestinal stricture, bowel obstruction or ileus). For these people, taking high fiber foods or supplements, such as flax or chia seeds, may worsen the obstruction.


  • High fiber contents of flaxseeds may cause the obstruction of the gastro intestinal tract, especially in people with a history of obstruction problems

Flaxseed side-effects related to absorption of medications and nutrients from supplements

Flaxseeds when taken together with oral medications or nutrient supplements (vitamins or minerals), may decrease the absorption of some of these components.

For maximum absorption, it is advisable to take those medications or supplements one hour before or two hours after flaxseeds.


  • Take oral medications or supplements one hour before or two hours after eating flaxseeds

Flaxseed side-effects related to blood pressure

Eating flaxseeds lowers the blood pressure considerably, and this effect may have undesirable effects, if the blood pressure is already too low.

This usually occurs in two situations: if hypotension and when anti-hypertensive drugs or herbal supplements (e.g. gymnema) are taken by people with hypertension.

However, more evidence is need.

NOTE: flaxseed oil doesn’t lower blood pressure.  (13)

For more on blood pressure reduction methods see “How to lower blood pressure?”

Contraindications when taken with anti-hypertensive drugs

While this is great news for hypertensive people that want to use natural treatment methods, it may present a problem when used in conjunction with blood lowering medication.


Flaxseeds may lower the blood pressure in people with hypotension.


  • Flaxseeds lower blood pressure, which may affect people with low blood pressure or on blood pressure lowering medications.

Flaxseed side-effects related to bipolar disease

One study showed that bipolar patients may experience increased episodes of mania and hypomania, when flaxseeds or flaxseed oil are taken together with mood stabilizers or lithium.

Mood-altering herbs, such as St. John’s wort, kava or valerian, may increase their effect in bipolar episodes, when taken together with flaxseeds or flaxseed oil. This hasn’t yet been clinically proven.


  • Flaxseeds may increase bipolar symptoms, if taken with mood stabilizers, lithium or mood-altering herbs.


You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.


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