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Both flaxseed and fish oil contain omega-3 fatty acids that, if consumed regularly, have been linked to a decreased risk of cardiovascular disease, stroke and high blood pressure 3. However, both are also capable of causing potentially severe side effects in certain users. These side effects range from mild digestive problems and allergic reactions to serious instances of internal bleeding and a possible increased risk for certain types of cancer. Consult your physician before using either fish oil or flaxseed on a regular basis 3.
Flaxseed is the small, reddish brown seed of the flax plant, also known by its scientific name Linum usitatissimum. Fish oil refers to the oils found in certain types of fish, mainly those that contain large amounts of omega-3 polyunsaturated fatty acids 3.
Sulfameth/Trimethoprim Side Effects
The Mayo Clinic reports that, at extremely high doses, fish oil can result in severe diarrhea that may cause dehydration if left untreated 3. Most of these unpleasant digestive system side effects can be avoided by taking flaxseed and fish oil with regular meals, drinking plenty of water and starting with very small amounts of both before working up to your supplementation goal 13.
Both flaxseed and fish oil can cause allergic reactions characterized by the sudden onset of an unexplained rash; itchy skin; difficulty breathing and swallowing; a wheezing sound while breathing; swelling in the lips, face or tongue; or general flu-like symptoms accompanied by a fever and body aches 3. If you experience any of these symptoms while using fish oil or flaxseed, stop using them immediately and seek medical assistance 3.
Vitamin K2 Toxicity
A number of medications can react with fish oil or flaxseed, resulting in unwanted side effects or causing the medicine to be much less effective 13. For fish oil, these medications include blood thinners like warfarin, birth control pills or other estrogen-containing drugs, beta-blocking drugs like carvedilol or propranolol and diruetics 3. For flaxseed, drugs that can cause interactions include any diabetes medications; any blood thinners, including aspirin; nonsteroidal anti-inflammatory drugs like Celebrex or ibuprofen; and any thrombolytic drugs.
According to Med TV, recent research into the omega-3 fatty acid ALA indicates that increased consumption of ALA may be linked to a higher incidence of prostate cancer. However, the research was conducted on animal products containing high amounts of ALA, not on plant-based compounds like flaxseed that contain large concentrations of ALA so it is not known definitively if regular flaxseed use can contribute to prostate cancer risk.
Using high doses of either flaxseed or fish oil is linked to an increased risk of internal bleeding 3. If you notice that you bruise abnormally easily; begin to to vomit blood or notice blood in your stool; have sudden changes in your speech or ability to see; or experience unexplained weakness in one of your limbs while using either substance as a dietary supplement, discontinue their use immediately.
Sulfameth/Trimethoprim Side Effects
Vitamin K2 Toxicity
Flaxseed Allergy Symptoms
The Toxicity of Linseed Oil
Can You Take Fish Oil With Lisinopril?
Oxygen & Severe Nosebleeds
Symptoms From High Platelets
Side Effects of IP6
Neo Citran Side Effects
Does Fish Oil Interact With Birth Control?
- Med TV: Flaxseed Side Effects
- The World's Healthiest Foods: Flaxseeds
- Drugs.com: Fish Oil
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- Ren GY, Chen CY, Chen GC, et al. Effect of flaxseed intervention on inflammatory marker C-reactive protein: A systematic review and meta-analysis of randomized controlled trials. Nutrients. 2016;8(3):136. doi:10.3390/nu8030136
- Okinyo-Owiti DP, Dong Q, Ling B, et al. Evaluating the cytotoxicity of flaxseed orbitides for potential cancer treatment. Toxicol Rep. 2015;2:1014-1018. doi:10.1016/j.toxrep.2015.06.011
- Lowcock EC, Cotterchio M, Boucher BA. Consumption of flaxseed, a rich source of lignans, is associated with reduced breast cancer risk. Cancer Causes Control. 2013;24(4):813-6. doi:10.1007/s10552-013-0155-7
- Ramos CI, Andrade de Lima AF, Grilli DG, Cuppari L. The short-term effects of olive oil and flaxseed oil for the treatment of constipation in hemodialysis patients. J Ren Nutr. 2015;25(1):50-6. doi:10.1053/j.jrn.2014.07.009
- Styrczewska M, Kostyn A, Kulma A, et al. Flax fiber hydrophobic extract inhibits human skin cells inflammation and causes remodeling of extracellular matrix and wound closure activation. Biomed Res Int. 2015;2015:862391. doi:10.1155/2015/862391
- Ibrügger S, Kristensen M, Mikkelsen MS, Astrup A. Flaxseed dietary fiber supplements for suppression of appetite and food intake. Appetite. 2012;58(2):490-5. doi:10.1016/j.appet.2011.12.024
- Cetisli NE, Saruhan A, Kivcak B. The effects of flaxseed on menopausal symptoms and quality of life. Holist Nurs Pract. 2015;29(3):151-7. doi:10.1097/HNP.0000000000000085
- National Center for Complementary and Integrative Health. Flaxseed and flaxseed oil. Updated September 2016.
- O'Keefe A, Kapur S, Rex G, Watson W. Flax seed allergy in children: an emerging allergen? Allergy Asthma Clin Immunol. 2010; 6(Suppl 2):P6. doi:10.1186/1710-1492-6-S2-P6
- Hackshaw-McGeagh LE, Perry RE, Leach VA, et al. A systematic review of dietary, nutritional, and physical activity interventions for the prevention of prostate cancer progression and mortality. Cancer Causes Control. 2015;26(11):1521-50. doi:10.1007/s10552-015-0659-4
- Carleton AJ, Sievenpiper JL, De Souza R, McKeown-Eyssen G, Jenkins DJ. Case-control and prospective studies of dietary α-linolenic acid intake and prostate cancer risk: a meta-analysis. BMJ Open. 2013;3(5). doi:10.1136/bmjopen-2012-002280
- Setayesh M, Sadeghifar AR, Nakhaee N, Kamalinejad M, Rezaeizadeh H. A topical gel from flax seed oil compared with hand splint in carpal tunnel syndrome: A randomized clinical trial. J Evid Based Complementary Altern Med. 2017;22(3):462-467. doi:10.1177/2156587216677822
- UW Medicine Anesthesiology and Pain Medicine. Medications to avoid before surgery.
- Mount Sinai Health Library. Flaxseed oil.
- Kajla P, Sharma A, Sood DR. Flaxseed--a potential functional food source. J Food Sci Technol. 2015;52(4):1857-71. doi:10.1007/s13197-014-1293-y
- Cockerell KM, Watkins AS, Reeves LB, Goddard L, Lomer MC. Effects of linseeds on the symptoms of irritable bowel syndrome: a pilot randomised controlled trial. J Hum Nutr Diet. 2012;25(5):435-43. doi:10.1111/j.1365-277X.2012.01263.x
- Kaithwas G, Majumdar DK. Effect of L. usitatissimum (flaxseed/linseed) fixed oil against distinct phases of inflammation. ISRN Inflamm. 2013;2013:735158. doi:10.1155/2013/735158
- Ward HA, Kuhnle GG, Mulligan AA, Lentjes MA, Luben RN, Khaw KT. Breast, colorectal, and prostate cancer risk in the European Prospective Investigation into Cancer and Nutrition-Norfolk in relation to phytoestrogen intake derived from an improved database. Am J Clin Nutr. 2010;91(2):440-8. doi:10.3945/ajcn.2009.28282
Michelle Kerns writes for a variety of print and online publications and specializes in literature and science topics. She has served as a book columnist since 2008 and is a member of the National Book Critics Circle. Kerns studied English literature and neurology at UC Davis.