The usual starting target for fish oil in cats is 30 mg of combined EPA and DHA per kg of body weight, once a day, given with a meal that contains fat. For a 4.5 kg cat that is 135 mg of EPA plus DHA, not 135 mg of fish oil. First changes in coat and skin take about 2 weeks, the full effect takes 4 to 6 weeks, and the dose is set by your vet, because weight, kidney disease, bleeding risk and diet all move the number. Stop 7 days before any surgery or dental cleaning.
30 mg/kg is a nutrient dose, not an oil dose
Fish oil is sold by total oil weight, and the useful part is only the EPA and DHA inside it. A 1000 mg capsule of a 30 percent concentrate carries 300 mg of EPA plus DHA, so a 4.5 kg cat at 30 mg/kg needs 135 mg, which is 450 mg of that oil, a bit less than half a capsule. A 500 mg capsule of a 60 percent product carries 300 mg in one soft gel, so half a capsule already overshoots. Always multiply the mg per capsule by the percentage on the label, then divide by the cat's weight to see what you are actually giving. Oils sold as 18 percent EPA and 12 percent DHA are the common 30 percent type.
Dose by body weight
| Cat weight (kg) | 20 mg/kg (mg) | 30 mg/kg (mg) | 40 mg/kg (mg) | Oil at 30% (mg) |
|---|---|---|---|---|
| 3.0 | 60 | 90 | 120 | 300 |
| 4.0 | 80 | 120 | 160 | 400 |
| 4.5 | 90 | 135 | 180 | 450 |
| 5.5 | 110 | 165 | 220 | 550 |
| 7.0 | 140 | 210 | 280 | 700 |
The 20 mg/kg column is a maintenance dose for a healthy cat with a dull coat. The 40 mg/kg column is where vets often go for joint or kidney support, and it is also where loose stool becomes common. Do not stack a 30 percent oil with a fish-based food and a separate omega supplement without adding the totals first.
Once a day, with fat, at the same time
Give the oil with the largest meal of the day, mixed into canned food at 78 percent moisture or into a teaspoon of a fatty food. Absorption of EPA and DHA depends on bile and pancreatic lipase, which the meal triggers, and giving oil to a fasting cat raises nausea and vomiting within 30 to 60 minutes. Splitting the dose into 2 meals is acceptable and often reduces loose stool, but once daily with food is simpler and works for most cats. Refrigerate the bottle after opening, keep capsules away from light, and use the opened bottle within 90 days; rancid oil smells sharp and causes vomiting within 1 to 2 hours.
Two weeks is the first checkpoint
EPA and DHA are incorporated into cell membranes over 2 to 3 weeks, which is why nobody can judge a fish oil at day 3. Skin and coat changes usually appear at 2 to 4 weeks, joint comfort at 4 to 6 weeks, and kidney protein loss trials run 6 months. Judge at 2 weeks on stool quality, appetite and vomiting, then at 6 weeks on coat and mobility. If nothing has changed at 6 weeks at a proper dose, more oil is not the answer; the problem is probably something else.
Bleeding risk and the 7 day stop
High-dose EPA shifts platelet aggregation and lengthens bleeding time a little. That is why the standard instruction is to stop fish oil 7 days before surgery, dental cleaning or a biopsy, and why a cat already on an NSAID, aspirin or an anticoagulant should not start fish oil without the vet's agreement. Signs to report at once are bleeding gums when brushing, nosebleeds, bruising on the belly, black tarry stool and blood in vomit. A cat with a platelet count below 50,000, a clotting disorder, or active bleeding should not take it at all. Dose is set by the vet, and the 30 mg/kg figure is a starting point, not a prescription.
What goes wrong at higher doses
Loose stool and fishy breath are the 2 most common complaints, usually above 60 mg/kg a day. Vomiting right after the dose often means the oil was given on an empty stomach or the product has oxidized. Weight gain is possible but small: 450 mg of oil is about 4 kcal, so it will not explain a 500 g gain in 2 months. Cats with pancreatitis should start at the low end or not at all, because added fat can trigger a flare. Very high doses can also displace vitamin E, so many vets pair 1 to 2 IU of vitamin E per gram of oil.
Fish oil is not the same as fish
Oily fish brings thiamine risk and mercury along with the omega-3. A cat eating a fish-heavy diet can develop thiamine deficiency within weeks, which is covered in fish and thiamine. Purified oil has the protein and the thiamine problem removed, which is the reason it is the safer route for a cat that needs a measured dose. If the cat is on a dry diet with 8 percent moisture, the oil adds fat without water, so watch stool and keep the fluid intake described in wet versus dry food in view.
When to call the vet instead
Call for vomiting that persists past 24 hours, diarrhea past 48 hours, refusal to eat for more than 24 hours, any bleeding, or a cat that is losing weight despite a good appetite. Fish oil does not treat itchy skin caused by fleas, ringworm or a food allergy, and it will not fix a cat that is grooming bald patches. It is an add-on for skin, joints and kidney support, never a replacement for treating the cause.
Five step starting plan
- Weigh the cat on a kitchen scale and write the kg down to 1 decimal.
- Read the label: mg of EPA plus DHA per capsule, not mg of oil.
- Divide 30 mg by kg, then work out how many capsules that is.
- Give with the biggest meal, once daily, same hour every day.
- Recheck at 2 weeks for stool, at 6 weeks for coat and mobility, and stop 7 days before any procedure.
Most cats settle at 30 mg/kg of EPA plus DHA a day and stay there for months. The number on the label, not the size of the capsule, decides whether the dose is right, and 7 days off before surgery is the one rule that is easy to forget.
Frequently asked
Is 30 mg/kg of fish oil or of EPA and DHA?
It is 30 mg of combined EPA and DHA per kg, not 30 mg of oil. A 4.5 kg cat therefore needs 135 mg of EPA plus DHA, which in a 30 percent concentrate is 450 mg of oil, slightly less than half of a 1000 mg capsule. Read the EPA and DHA mg per capsule before dosing.
Why stop fish oil 7 days before surgery?
High-dose EPA reduces platelet clumping and lengthens bleeding time slightly, which matters during surgery, dental extraction or a biopsy. Seven days is roughly 3 to 5 half-lives of the fatty acid in cell membranes and enough for platelet function to return to baseline. Restart only when the vet says healing is on track.
How long until I see a difference in the coat?
About 2 weeks for the first change and 4 to 6 weeks for a clear one, because EPA and DHA need 2 to 3 weeks to settle into cell membranes. Judge stool and appetite at 2 weeks, coat and mobility at 6 weeks, and do not raise the dose before that if the cat is already at 30 mg/kg.