Are there any side effects of eating flaxseed?

December 26, 2025 •

5 min read

One tablespoon of ground flaxseed contains approximately 2 grams of dietary fiber, a nutrient that, in high amounts or without adequate water, can trigger notable digestive side effects. While a superfood praised for its omega-3s, lignans, and fiber, flaxseed is not without potential downsides that warrant caution. Understanding these risks is key to safe consumption.

Quick Summary

Eating flaxseed, though beneficial, can cause digestive issues like bloating, gas, and cramps, especially if intake is increased too quickly or without enough fluid. High fiber can affect medication absorption and may cause blockage in rare cases. Flaxseed also has phytoestrogens that can impact hormone-sensitive conditions and may interact with certain medications.

Key Points

Digestive Distress: Sudden increases in fiber from flaxseed can cause bloating, gas, abdominal cramps, and changes in bowel habits like diarrhea or constipation.
Medication Interference: Flaxseed's fiber can block the absorption of oral medications, while its properties can interfere with blood thinners, blood sugar drugs, and hormone therapies.
Hormonal Effects: The phytoestrogens in flaxseed may affect individuals with hormone-sensitive conditions like breast cancer, PCOS, or those who are pregnant or breastfeeding.
Bowel Obstruction: Insufficient fluid intake combined with high fiber can cause intestinal blockage, a particular risk for those with pre-existing digestive issues.
Toxicity of Raw Flaxseed: Raw or unripe flaxseeds contain potentially toxic cyanogenic compounds and should never be consumed.
Allergic Reactions: Though uncommon, allergic reactions to flaxseed are possible and can range from mild itching to severe anaphylaxis.
Ground vs. Whole Seeds: To ensure maximum nutrient absorption and prevent digestive issues from undigested shells, it is best to consume flaxseed in its ground form.

Common Digestive Issues

Flaxseed is renowned for its high fiber content, which can improve digestion and promote regularity. However, a rapid increase in fiber intake can shock the digestive system, leading to uncomfortable side effects. Bloating, gas, and abdominal cramping are among the most common complaints. For some, the laxative effect of soluble fiber can also cause diarrhea, while for others, especially those who don't drink enough water, it can cause or worsen constipation.

Managing Digestive Discomfort

To avoid these issues, it is important to introduce flaxseed gradually into your diet and ensure proper hydration. Start with a small amount, such as one teaspoon per day, and slowly increase the serving size over a few weeks as your body adjusts. Crucially, always consume flaxseed with plenty of water. The soluble fiber in flaxseed absorbs water to form a gel, which helps move waste through the digestive tract. Without sufficient fluid, this bulking agent can cause intestinal obstruction.

Tips for Minimizing Side Effects

Start small: Begin with a low dose and increase gradually.
Stay hydrated: Drink extra water throughout the day to help the fiber pass smoothly.
Choose ground over whole: Ground flaxseed is easier to digest and its nutrients are more readily absorbed. Whole seeds often pass through the body undigested.
Combine with other foods: Mix flaxseed into smoothies, oatmeal, or yogurt to help spread out the fiber intake.

Potential for Drug Interactions

Flaxseed can interact with certain medications, primarily due to its high fiber content and ability to affect blood sugar and clotting.

Medications that slow blood clotting: Flaxseed and flaxseed oil can have blood-thinning effects. When taken with anticoagulant or antiplatelet drugs like warfarin or aspirin, it can increase the risk of bruising and bleeding. It's recommended to stop using flaxseed at least two weeks before any scheduled surgery.
Diabetes medications: Flaxseed may lower blood sugar levels. Taking it alongside diabetes medications could cause blood sugar to drop too low. Monitoring blood sugar closely is advised.
Oral medications: The fiber in flaxseed can interfere with the absorption of other medicines. To prevent this, it's best to take flaxseed at least 1-2 hours before or after taking other oral medications.
Estrogen-dependent treatments: Because flaxseed contains phytoestrogens, it may affect the effectiveness of hormone therapies or oral contraceptives.

Hormonal Effects and Precautions

Flaxseed contains lignans, which are compounds that can act as phytoestrogens, or plant-based compounds that mimic the hormone estrogen. This can be a benefit for some but a concern for others.

Hormone-sensitive conditions: Individuals with hormone-sensitive conditions, such as certain types of breast or ovarian cancer, endometriosis, or PCOS, should consult their doctor before using flaxseed. The phytoestrogens might potentially exacerbate these conditions.
Pregnancy and breastfeeding: Due to the estrogen-like effects and a lack of conclusive evidence on safety, pregnant and breastfeeding women are often advised to avoid large amounts of flaxseed and flaxseed oil.

Other Risks and Considerations

While less common, other risks associated with flaxseed consumption exist.

Allergic reactions: Though rare, allergic reactions to flaxseed can occur. Symptoms can include itching, rashes, nausea, vomiting, or diarrhea. In severe cases, anaphylaxis is possible.
Cyanogenic compounds: Raw or unripe flaxseeds contain cyanogenic compounds that can be toxic. For this reason, flaxseeds should only be eaten ripe and roasted, or in their ground form.
High triglycerides: Some types of flaxseed, specifically partially defatted flaxseed, may increase triglyceride levels. Those with high triglycerides should consult their doctor before consumption.

Whole vs. Ground Flaxseed

The form in which flaxseed is consumed significantly impacts its benefits and potential for side effects. For most people, ground flaxseed is the better option.


Feature	Whole Flaxseed	Ground Flaxseed
Digestion	Often passes through the digestive system undigested, potentially leading to lower nutrient absorption.	Easily digested, allowing for greater absorption of omega-3s, fiber, and lignans.
Nutrient Absorption	Limited absorption of beneficial compounds like alpha-linolenic acid (ALA), lignans, and fiber.	Optimal absorption, as the ground shell releases the nutrients.
Digestive Impact	Can cause digestive issues if not chewed thoroughly, and poses a greater risk of intestinal blockage if consumed in large amounts without enough water.	Still requires adequate hydration to prevent bloating and gas, but carries less risk of blockage due to smaller particle size.
Shelf Life	Longer shelf life, as the hard outer shell protects the oil inside from oxidation.	Shorter shelf life; the exposed oils can go rancid more quickly. Should be stored in an airtight container in the refrigerator.

Conclusion

Flaxseed offers significant health benefits, including high fiber and omega-3 content, but it is not without potential side effects. The most common issues relate to digestive discomfort, which can often be managed by starting with small doses, drinking plenty of water, and choosing ground over whole seeds. More serious risks involve potential drug interactions, especially with blood thinners and diabetes medications, and hormonal effects that warrant caution for individuals with hormone-sensitive conditions, pregnant women, and those who are breastfeeding. It is essential to consult a healthcare provider before adding significant amounts of flaxseed to your diet, particularly if you have underlying health conditions or take other medications. For further information, consult the National Center for Complementary and Integrative Health Guide to Flaxseed. NCCIH Guide to Flaxseed

Expert Recommendations at a Glance

Start low and go slow: Begin with a small amount of ground flaxseed and gradually increase your intake to let your body adjust.
Drink up: Always consume flaxseed with plenty of fluids to prevent constipation and the risk of bowel obstruction.
Talk to your doctor: Discuss flaxseed intake with your healthcare provider, especially if you have pre-existing conditions, are pregnant, or are taking medications.
Store correctly: Keep ground flaxseed in an airtight container in the refrigerator to prevent rancidity and preserve nutrients.
Choose the right form: Opt for ground flaxseed to maximize nutrient absorption and minimize digestive discomfort compared to whole seeds.
Avoid raw: Never eat raw or unripe flaxseeds, as they contain potentially toxic compounds.

Following these guidelines can help you safely incorporate this nutrient-dense seed into your diet while mitigating potential side effects.

Frequently Asked Questions

Flaxseed is high in fiber, and consuming too much too quickly, especially without enough water, can cause digestive upset. The fiber ferments in the gut, which can produce excess gas and lead to bloating.

No, you should never eat raw or unripe flaxseed. It contains potentially toxic cyanogenic compounds. Always consume flaxseed that has been processed, such as ground, toasted, or baked into products.

Flaxseed can interfere with the absorption of oral medications due to its high fiber content. For specific drugs like blood thinners or diabetes medications, it can also have additive effects, potentially increasing the risk of bleeding or causing blood sugar to drop too low.

People with certain hormone-sensitive conditions (like some cancers), those on blood thinners, or women who are pregnant or breastfeeding should consult a doctor before consuming flaxseed. Individuals prone to bowel obstruction should also be cautious.

For optimal nutrient absorption and digestibility, it is better to eat ground flaxseed. The tough outer shell of whole seeds can pass through your system undigested, meaning you won't get all the benefits.

Most experts agree that 1 to 2 tablespoons of ground flaxseed per day is a healthy and safe amount for adults in a balanced diet. Starting with a smaller amount and increasing slowly can help prevent digestive side effects.

Yes. Flaxseed oil lacks the fiber content of the meal, so it doesn't cause the same digestive bulk issues. However, it still contains omega-3s and can interact with medications that affect blood clotting or blood sugar. It's also sensitive to heat and light and can go rancid easily.