Two source families cover every cat: marine fish oil, and non-fish oils such as algal, krill, and green-lipped mussel. What decides the outcome is not the bottle design but the combined EPA+DHA figure, the volume that delivers it, and the ceiling you should not cross. Most cats need 20 to 30 mg/kg EPA+DHA per day for general support, 40 to 60 mg/kg for skin and joint work, and 60 to 100 mg/kg only under veterinary direction. A 4 kg cat therefore lands between 80 and 400 mg per day, which is 0.3 to 1.3 mL of a standard 300 mg/mL fish oil.
Why the EPA+DHA number beats the oil name
Salmon, anchovy, sardine, and cod liver oil are sold as if they were 4 different products. In practice they sit in 2 concentration tiers. A typical anchovy oil carries 300 mg EPA+DHA per 1 mL, or 30%. A salmon oil carries 180 to 220 mg, or 18% to 22%. Feed the same 1 mL and you deliver a 40% difference in active fatty acid. Cod liver oil adds a second problem: 5 mL supplies roughly 4500 IU vitamin A and 450 IU vitamin D, so a cat given 3 mL a day for 8 weeks can approach the vitamin A ceiling before it reaches a useful EPA dose. Use cod liver oil as a vitamin source, never as the omega-3 source.
5 forms and what 1 mL actually delivers
The table converts label claims into the number you dose with. The last column is the volume a 4 kg cat needs at 40 mg/kg, which is 160 mg of EPA+DHA.
| Source | EPA+DHA per 1 mL | mL per day at 40 mg/kg | Cost per 30 days | Best use |
|---|---|---|---|---|
| Anchovy or sardine oil, triglyceride form | 300 mg (30%) | 0.53 mL | 35 to 60 CNY | Default choice, 12-month shelf life unopened |
| Salmon oil | 220 mg (22%) | 0.73 mL | 60 to 90 CNY | Picky cats, stronger smell |
| Krill oil | 240 mg (24%) | 0.67 mL | 120 to 180 CNY | Phospholipid form, 1 drop dosing |
| Algal oil, DHA-rich | 400 mg DHA + 40 mg EPA | 0.36 mL | 150 to 220 CNY | Fish allergy, 100% plant source |
| Flaxseed oil | 0 mg EPA+DHA, 530 mg ALA | Not usable | 20 to 35 CNY | Do not count as a cat source |
The daily ceiling, and 3 signs you passed it
Most veterinary nutrition references place a practical ceiling near 100 to 150 mg/kg EPA+DHA per day, which is 400 to 600 mg for a 4 kg cat. Above that, 3 things show up within 2 to 3 weeks: loose stool or vomiting within 30 minutes of the dose, a fishy odor on the coat and breath, and a calorie problem, because 1 mL of oil adds 9 kcal and a 4 kg cat needs only 200 to 250 kcal a day. At 2 mL of oil you have spent 18 kcal, or 8% of the daily budget. A cat with a bleeding disorder, one scheduled for surgery within 14 days, or one on warfarin-type therapy needs the dose set by a veterinarian, not by a bottle label.
4 handling rules that decide potency
Polyunsaturated fat oxidizes, and oxidized oil is worse than no oil. Buy a bottle that states the EPA+DHA figure in mg per mL, not only the oil weight, and check for a batch date within 6 months. Store it in the refrigerator at 4 °C, not in a cupboard at 25 °C, and keep the bottle dark. Smell it at week 4: fresh fish oil smells faintly of the sea, rancid oil smells like paint or old nuts, and a cat that suddenly refuses a previously accepted oil is often detecting rancidity before you do. A 250 mL bottle lasts a 4 kg cat about 8 months at 1 mL a day, which is longer than most oils stay fresh, so buy 100 to 120 mL sizes instead.
When algal or krill oil wins
Algal oil is the answer for 2 groups: cats with confirmed adverse reaction to fish, and households that will not handle fish products. It is also the only plant source with a meaningful EPA+DHA figure, because cats lack the enzyme efficiency to convert ALA; expect under 5% conversion from flaxseed, which is why a 530 mg ALA dose yields well under 30 mg of usable EPA+DHA. Krill oil wins on volume, since 1 drop is about 25 mg and you can titrate a 2 kg cat at 20 mg/kg without a syringe. Both cost 2 to 4 times more per 100 mg of EPA+DHA than anchovy oil, so pay for them only when the reason applies.
Reading a label in 30 seconds
Skip the front panel. Find 3 numbers on the back: total omega-3 per serving, EPA per serving, DHA per serving. If only total omega-3 is printed, assume 60% of it is EPA+DHA and dose conservatively. Check whether the serving is 1 mL, 1 pump, or 1 capsule, because a 500 mg capsule of 30% oil holds only 150 mg. Confirm the form: triglyceride (TG) absorbs with food, ethyl ester (EE) needs fat in the same meal, and re-esterified triglyceride behaves like TG at a higher price. A product listing 1000 mg fish oil and no EPA+DHA figure is a 300 mg product at best, and 1 in 3 budget bottles tested by their own sellers falls below the label claim.
4 steps to start and stay on track
1. Weigh the cat and set the dose in mg, not in pumps: kg × 40 mg for joint or skin goals. 2. Divide by the mg per mL on the label to get mL, and measure with a 1 mL syringe for anything under 1 mL. 3. Split the daily amount into 2 meals to cut loose stool, and mix into wet food at 30 °C or below. 4. Log body weight and stool score at week 0, 4, and 8, and stop the product if the coat odor or stool change appears. Pair the oil with a protein plan from protein source quality so the added fat does not push the diet past its calorie target, and check any fish-flavored treat against the toxicity list before it becomes a habit.
Frequently asked
Can I just use cod liver oil as the omega-3 source?
Not for daily use. Five milliliters of cod liver oil carries about 4500 IU of vitamin A and 450 IU of vitamin D, so a 4 kg cat dosed for EPA would hit the vitamin A ceiling long before the fatty acid target. Use it as a vitamin D source at 1 to 2 mL a week and buy a plain anchovy or sardine oil for EPA and DHA.
How long before I see a change in coat or stiffness?
Most owners notice coat change at 4 to 6 weeks and gait change at 6 to 8 weeks, which is why a 8 week trial is the standard before judging a product. Photograph the cat walking away from you at week 0 and week 8, weigh monthly, and stop if stool loosens or the coat smell turns fishy within 2 to 3 weeks of starting.
Is flaxseed oil a cheaper substitute?
No. Flaxseed oil supplies ALA, not EPA or DHA, and cats convert under 5% of it, so 530 mg of ALA yields well under 30 mg of usable fatty acid. Krill or algal oil costs more per bottle but delivers a dose you can actually measure, and algal oil is the only plant option with a real EPA+DHA figure.