What are the side effects of rancid flaxseed oil?

November 24, 2025 •

3 min read

According to research, consuming oxidized oils can lead to the formation of harmful free radicals in the body. This is a critical concern, as understanding what are the side effects of rancid flaxseed oil can help protect your health, since this otherwise beneficial supplement becomes toxic when spoiled.

Quick Summary

Oxidized flaxseed oil can cause digestive distress, inflammation, and cellular damage from free radicals. Long-term consumption of rancid oil may contribute to chronic diseases and diminish nutritional benefits.

Key Points

Long-term Risks: Regular consumption of rancid flaxseed oil can lead to chronic diseases, including increased risk of atherosclerosis, cancer, and neurodegenerative disorders.
Digestive Issues: Immediate side effects can include nausea, cramping, and bloating due to digestive distress.
Nutrient Depletion: The oxidation process destroys the beneficial omega-3 fatty acids and fat-soluble vitamins (B and E), eliminating the oil's health advantages.
Free Radical Damage: Rancid oil produces harmful free radicals that cause oxidative stress, damaging cells throughout the body.
How to Identify: Spoiled oil has a characteristic bitter, sharp, or 'paint-like' taste and a fishy or stale odor, along with a cloudy appearance.
Proper Storage is Key: To prevent rancidity, store flaxseed oil in a dark, cool place (preferably the refrigerator) and keep the bottle tightly sealed.

The Chemical Process of Rancidity in Flaxseed Oil

Flaxseed oil is highly susceptible to rancidity due to its high concentration of polyunsaturated fats, especially omega-3 fatty acid (alpha-linolenic acid). This oxidation process occurs when the oil is exposed to light, heat, and air. This triggers a chain reaction where oxygen attacks the double bonds in the fatty acids, creating free radicals. These free radicals then create harmful secondary oxidation products like aldehydes and ketones, which are responsible for the unpleasant odor and taste. While consuming a small amount of slightly rancid oil may not cause immediate illness, regular consumption poses significant long-term health risks.

Digestive System Side Effects

The most immediate and common side effects of consuming rancid flaxseed oil are related to the digestive system. The breakdown products from oxidation can irritate the stomach and intestines, leading to a variety of unpleasant symptoms.

Nausea and Upset Stomach: The bitter taste and pungent smell can cause an immediate feeling of sickness.
Digestive Distress: Common symptoms include cramping, gas, and bloating.
Diarrhea: In some cases, consumption can lead to diarrhea.

Systemic and Long-Term Health Risks

Beyond immediate digestive issues, repeated consumption of rancid oils is a far greater concern due to its systemic effects on the body. The free radicals and other toxic compounds generated during oxidation can damage cells and contribute to chronic conditions over time.

Chronic Inflammation: Oxidized fats are pro-inflammatory and can increase inflammation throughout the body. Chronic inflammation is a driver of numerous diseases.
Cellular Damage: Free radicals produced during rancidity cause oxidative stress, which leads to cell damage and accelerates the aging process.
Atherosclerosis and Heart Disease: Studies suggest a link between consuming oxidized oil and the development of atherosclerosis (thickening of artery walls), which increases the risk of heart disease.
Cancer and Other Degenerative Diseases: The toxic compounds found in rancid oils have been studied for their potential carcinogenic and mutagenic properties. Research has linked these compounds to an increased risk of cancer, neurological disorders, and diabetes over time.
Nutrient Depletion: The oxidation process can destroy valuable fat-soluble vitamins, such as vitamins B and E, reducing the oil's nutritional value and potentially leading to vitamin deficiencies.

Comparison Table: Fresh vs. Rancid Flaxseed Oil


Feature	Fresh Flaxseed Oil	Rancid Flaxseed Oil
Appearance	Clear, golden-yellow color	Cloudy, murky, or discolored
Smell	Mild, pleasant, slightly nutty or earthy	Strong, unpleasant, often described as paint-like, stale, or fishy
Taste	Mild, nutty, and clean	Bitter, sharp, or acrid
Nutritional Value	High in beneficial omega-3 fatty acids	Significantly diminished nutritional benefits
Health Effects	Anti-inflammatory and cardio-protective	Pro-inflammatory and potentially toxic compounds

How to Prevent Flaxseed Oil from Going Rancid

Store Properly: Keep flaxseed oil in a cool, dark place, preferably the refrigerator. Exposure to light and heat are the main culprits of oxidation.
Keep it Airtight: The bottle should always be tightly sealed to minimize oxygen exposure.
Purchase Small Batches: Buying smaller bottles ensures you finish the oil before it has a chance to turn rancid.
Check the Expiration Date: Always check the expiration date and adhere to it, even with proper storage.
Do Not Heat: Avoid using flaxseed oil for cooking, as high heat accelerates the oxidation process significantly. It is best used raw, such as in salad dressings or smoothies.

Conclusion

While fresh flaxseed oil offers numerous health benefits, the consumption of rancid flaxseed oil carries significant health risks. The oxidation process destroys beneficial nutrients and creates harmful free radicals and toxic compounds that can lead to digestive issues, chronic inflammation, and an increased risk of long-term diseases like atherosclerosis and cancer. By understanding the signs of spoilage and practicing proper storage, you can avoid these adverse side effects and ensure you are getting the full nutritional benefits from your flaxseed oil. If in doubt about the freshness of your oil, it is always safest to discard it. For more information on lipid oxidation, consult reliable scientific sources.

Frequently Asked Questions

Rancid flaxseed oil has a distinctively unpleasant, stale, and fishy odor, which some people describe as smelling like old paint or varnish.

While it won't cause immediate food poisoning-like symptoms, consuming a small amount of rancid oil can cause nausea or an upset stomach due to its bitter taste. Regular, long-term consumption poses the greatest health risk.

The primary risk is the formation of harmful free radicals during the oxidation process, which can cause long-term cellular damage and contribute to the development of chronic diseases.

Flaxseed oil should be stored in a cool, dark place, such as a refrigerator, in a tightly sealed, opaque bottle to protect it from heat, light, and air.

While fresh flaxseed oil is an edible supplement, its oxidized form is the same as linseed oil used in varnishes. While some use it for woodworking, it is not recommended for ingestion in any form, as it contains toxic compounds.

Yes, as flaxseed oil becomes rancid through oxidation, its nutritional value diminishes. The beneficial omega-3 fatty acids and fat-soluble vitamins are degraded and destroyed.

To check for freshness, pour a small amount into a spoon and smell it. A clean, nutty, or earthy scent indicates freshness. A pungent, fishy, or paint-like odor means it is rancid and should be discarded.