Side Effects of Fish Oil: Does Fish Oil Thin The Blood?

I often say that it’s not that fish oil has side effects, it’s more that it just has effects. Side effects is the wrong way to think of it.

That said, let’s address exactly what those fish oil effects are and specifically if fish oil thins the blood.

Is Fish Oil a Blood Thinner? 

omega 3 side effects
Fish oil blood thiner

Fish oil, or better known as omega-3 fish oil, is a blood thinner, which is a great thing. It’s also an anti-inflammatory agent. As we grow older, blood flow in our body can become constricted. This is why many turn to fish oil supplementation as they get older to thin the blood moderately and support cardiovascular health. (1

Why It’s Important to Support Healthy Red Blood Cells As We Age

Red blood cells are responsible for carrying oxygen from our lungs to the rest of our body. (2) Red blood cells need to pass through the capillary beds to ensure oxygen gets to every metabolically active cell in the body. It’s important for waste removal as well as for delivering oxygen for energy production. Red blood cells really do a whole lot more, but those are two of their main and very important functions. 

It’s also important to talk about how blood moves through the capillaries. They may be as small as a single red blood cell — it’s not the heartbeat that controls the movement of blood in the capillaries, but “vasomotion.” It’s a muscular action of the capillary; therefore, blood needs to be a little thinner in order to pass through. (3)

Fish oil also acts as a natural anticoagulant, meaning that it helps prevent our blood from clotting. (4)

Important Note: Since omega-3s act as a moderate blood thinner, let your doctor know if you are on Warfarin or Coumadin before taking a fish oil supplement. Due to its blood-thinning properties, you will also want to speak with your doctor about stopping fish oil before any upcoming surgeries. 

Other Fish Oil Side Effects 

While fish oil does have a blood-thinning side effect, a few of the other, what I like to call “effects” of fish oil may be loose stools when first starting out. If you notice this, start slowly and work your way up to a dose that’s comfortable and well-tolerated. 

Side effects, if any, are often very mild. (5) But, if this is your first time taking supplements, you may want to build up to your digestive system tolerance level. After a week, you should be tolerating omega-3s very well, as long as the product is fresh and pure.

Additional Reasons to Consider Fish Oil Supplementation

omega 3 supplements 1
Omega 3 supplements side effects

No matter how old you are, there are other important reasons to consider fish oil supplementation. When taking omega-3s, we are trying to help combat the overdose of omega-6 in our diets, which causes systemic inflammation. 

This is a big issue in the US, where so many consume inflammatory foods, including trans and hydrogenated fats. We are getting far too many omega-6 fats and not nearly enough omega-3s

Taking a high-quality fish oil is a great way to help support a better balance when combined with a healthy diet. Cut back on processed foods and learn the benefits of real live food. You’ll notice the perimeter of the supermarket is where the real food lives so stay away from the packages of “food-like substances” in the aisles.

Research has also shown that omega-3s have a ton of benefits, such as supporting brain, skin, and eye health. (6, 7, 8, 9) If you want to find out more about omega-3s in general, take a look at our Ultimate Guide to Omega-3.

Final Thoughts

Supporting health is all about balance. While omega-3s are an important piece, don’t forget other aspects of health and wellness, something that I like to call our MEDS. Mindfulness, Exercise, Diet, and Sleep. Optimize them all to live a healthy and happy life. 


  1. National Center For Complementary and Integrative Health. Omega-3 Supplements: In Depth.
  2. American Red Cross. Red Blood Cells Why Are They Important.
  3. Lumen Learning. Structure & Function of Blood Vessels.
  4. Medical News Today. What Side Effects Can Fish Oil Cause?
  5. National Center For Complementary and Integrative Health. Omega-3 Supplements: In Depth.
  6. Emma Derbyshire. Brain Health Across the Lifespan: A Systemic Review on the Role of Omega-3 Fatty Acid Supplements.
  7. Harvard Health Publishing. Omega-3 For Your Eyes.
  8. Tse-Hung Huang, Pei-Wen Wang, […], and Jia-You Fang. Cosmetic and Therapeutic Applications of Fish Oil’s Fatty Acids on the Skin.
  9. Jung-Il Kang, Hoon-Seok Yoon, […], and Hee-Kyoung Kang. Mackerel-Derived Fermented Fish Oil Promotes Hair Growth by Anagen-Stimulating Pathways.

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