What Are the Side Effects of Fish Oil for Dogs? Full Guide
Fish oil is a favorite among dog owners looking to support their pup’s health naturally.
But just because it’s natural doesn’t mean it’s always safe.
Before adding fish oil to your dog’s daily routine, it’s important to understand how it works—and what can go wrong if it’s not used the right way.
Let’s walk through the real side effects of fish oil for dogs and how to give it safely so your furry friend gets the benefits without the risks.
Omega-3 fatty acids, primarily EPA and DHA, are abundant in fish oil.
These two nutrients support brain function, reduce inflammation, and help dogs maintain healthy skin and coats (1).
That’s why many vets recommend it for dogs with itchy skin, stiff joints, or chronic inflammation.
It’s also considered a natural, gentle alternative to pharmaceutical drugs.
But like anything, too much of a good thing can backfire.
Potential Side Effects of Fish Oil in Dogs
If your dog starts taking fish oil and suddenly has loose stools or throws up, it’s probably not a coincidence.
Gastrointestinal Problems
GI issues like diarrhea, vomiting, and excessive gas are some of the most common side effects of fish oil (2).
They usually happen if the dose is too high or introduced too quickly.
Some dogs are also sensitive to oils in general, especially if their diet wasn’t high in fats before.
Try lowering the dose or switching to a more purified fish oil formula to see if that helps.
Allergic Reactions
It’s rare, but some dogs are actually allergic to fish.
If you notice itching, rashes, red patches, or swelling after giving fish oil, stop right away.
Even if your dog isn’t allergic to fish itself, some supplements contain additives or flavors that can trigger a reaction.
Always check the label and choose a clean, single-ingredient product.
Weight Gain
Fish oil is a fat.
That means it’s full of calories—roughly 40 calories per teaspoon, depending on the brand.
If you’re adding fish oil to your dog’s food without adjusting the rest of their calories, the scale could creep up over time.
Weight gain can worsen arthritis and strain the heart, especially in older dogs.
Keep an eye on their body condition and talk to your vet about proper dosing.
Blood Clotting Issues
Fish oil can thin the blood (3).
That might be helpful for some heart conditions, but it also means your dog might bleed more easily.
If your pup is going in for surgery or is already on medication that affects clotting, fish oil could make things riskier.
Always check with your vet if your dog is on any prescriptions or has a history of bleeding problems.
Slower Healing
Omega-3s reduce inflammation, but that can be a double-edged sword.
In some cases, fish oil might slow down wound healing (4) or recovery after surgery because it dampens the body’s natural inflammatory response.
This side effect isn’t common, but it’s worth noting if your dog is recovering from an injury.
What Affects How Your Dog Reacts to Fish Oil?
Most side effects come down to one thing: dose.
The ideal amount of fish oil depends on your dog’s weight and health status.
In general, vets recommend around 20–55 mg of combined EPA and DHA per pound of body weight per day (5).
Start with the lower end of that range and gradually increase if needed.
Don’t just eyeball it—measuring accurately makes a big difference.
The Quality of the Supplement
Not all fish oil is created equal.
Some products use low-quality oil with added flavors or preservatives that can upset your dog’s system.
Look for brands that use wild-caught fish, third-party testing, and no unnecessary additives.
A clean, high-quality product is much less likely to cause problems.
Your Dog’s Unique Health
Every dog is different.
Older dogs, those with chronic diseases, or pups on medications may react differently to supplements.
Even if your friend’s dog thrives on fish oil, your dog might not.
That’s why it’s smart to check in with your vet before starting anything new.
They’ll help you personalize the dose and choose a product that works for your dog’s specific needs.
How to Use Fish Oil Safely
Consult your veterinarian before beginning.
Based on your dog's size, age, and overall health, they will determine the appropriate dosage.
Start with a low dosage and increase it over a few weeks if you're doing it by yourself.
Watch for Changes
Any new supplement should come with a bit of observation.
Pay attention to your dog’s poop, energy level, appetite, and skin condition.
If something seems off, lower the dose or stop completely and call your vet.
Side effects usually go away once you stop the supplement, but it’s always better to catch them early.
Choose Wisely
Pick a fish oil made specifically for pets.
Human supplements often contain xylitol or added vitamins that are dangerous to dogs.
Stick to products that clearly list the amount of EPA and DHA per serving, and avoid anything with vague “proprietary blends.”
Look for purity certifications like IFOS or NSF.
Your dog deserves the good stuff.
Final Thoughts
Think of fish oil like sunlight.
A little can do wonders—but too much, or the wrong kind, and you’ll get burned.
When used correctly, fish oil can be a powerful tool for supporting your dog’s health, especially if they have joint stiffness, allergies, or dull coats.
But the key is caution.
Start slow. Use the right dose. Choose high-quality.
And always listen to your dog’s body.
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Check out Wild Foods’ ultra-pure, sustainably sourced fish oil.
Your pup will thank you for it.
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FAQs about the Side Effects of Fish Oil for Dogs
Can I give my dog fish oil every day?
Yes, but only if the dose is right. Daily use is common, but talk to your vet before making it a long-term habit.
What happens if I give my dog too much fish oil?
They might get diarrhea, vomit, gain weight, or show signs of bleeding. Start slow and monitor closely.
Can fish oil make my dog itchy?
It is rare, but it could happen if your dog is allergic to fish or any of the chemicals in the supplement. Changing to a pure recipe is often advantageous.
Is fish oil good for dogs with cancer?
It might help, but it depends on the dog and their treatment plan. Always ask your vet first—fish oil can interact with medications.
What’s the best kind of fish oil for dogs?
Look for wild-caught, third-party tested oil made just for dogs. Avoid anything with additives, preservatives, or mystery ingredients.
Related Studies
1. Title: Therapeutic Effect of EPA/DHA Supplementation in Neoplastic and Non-Neoplastic Diseases in Dogs and Cats
This systematic review found that EPA and DHA supplementation provides therapeutic benefits for canine allergic dermatitis, haircoat disorder, and osteoarthritis, supporting their use in managing skin and joint health in dogs.
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC8193331/
2. Title: Fish oil: what the prescriber needs to know
This review highlights that intolerance to fish oil in dogs can manifest as gastrointestinal symptoms such as diarrhea, repeating taste, and aversion to odor, with about 15% of subjects experiencing these effects.
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC1526555/
3. Title: Uses and benefits of omega-3 ethyl esters in patients with hypertriglyceridemia
This study discusses the antithrombotic effects of omega-3 fatty acids and notes that high doses can prolong bleeding time, highlighting the need for monitoring in patients (including dogs) receiving anticoagulant therapy.
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC3004594/
4. Title: ω-3 fatty acids effect on wound healing
This study found that dietary supplementation with EPA/DHA led to a nonsignificant delay in wound healing in a canine model, potentially due to increased local inflammation in the early healing phase.
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC2967211/
5. Title: Canine Fish Oil Dosing Chart – Veterinary Teaching Hospital
This resource provides evidence-based maximum daily dosages of EPA/DHA for dogs with osteoarthritis, recommending 310 mg/kg^0.75 per day, with adjustments for individual tolerance.
Link: https://vetmedbiosci.colostate.edu/vth/services/orthopedic-medicine/fish-oil-dosing/
