Flaxseed Side Effects

Due to the high fiber content in flaxseed, side effects could occur if you do not start with a low dose and increase it gradually. Potential side effects include bloating, abdominal pain, and constipation. There are also some potentially serious side effects of flaxseed that require immediate medical attention, such as an allergic reaction, any signs of bleeding, and severe constipation or an intestinal blockage.

Does Flaxseed Cause Side Effects?

Even though flaxseed is a natural product, it is not free of side effects. For most people, these side effects are merely bothersome. However, flaxseed can theoretically cause serious side effects as well, especially at very high intakes.
Flaxseed is not exactly the same as flaxseed oil, which does not contain some of the active components of flaxseed. The information in this article does not apply to flaxseed oil.

Bothersome Side Effects of Flaxseed

Flaxseed is high in fiber and can cause the usual fiber side effects. Many people may experience digestive flaxseed side effects, such as:
Just as with any fiber, starting flaxseed at a low dose and increasing it slowly may help prevent many of these digestive side effects.

Serious Flaxseed Side Effects

There are a number of side effects with flaxseed that you should report to your healthcare provider and which may indicate that you should stop taking flaxseed. These include, but are not limited to:
  • Severe constipation or an intestinal blockage (which is possible if you take flaxseed without enough water)
  • Any signs of bleeding (which may theoretically occur with high doses of flaxseed), such as:


    • Easy bruising or bleeding
    • Black, tarry stools; bright red blood in the stool; or vomiting of blood (signs of gastrointestinal bleeding)
    • Signs of a hemorrhagic stroke (bleeding in the brain), such as vision or speech changes; weakness or numbness in an arm or leg; or a severe headache


  • Signs of an allergic reaction, including an unexplained rash, hives, itching, unexplained swelling, wheezing, or difficulty breathing or swallowing.
Flaxseed contains alpha-linolenic acid (ALA), and research suggests that diets high in ALA from meat and dairy may increase the risk of prostate cancer. However, this does not seem to apply to plant-based ALA, such as that found in flaxseed.
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