The label problem
A '1000mg fish oil' softgel usually contains only 300-400mg of actual EPA and DHA, the two omega-3s with the evidence behind them. The rest is other fatty acids and filler oil. This is the single most common reason people take fish oil for months and feel nothing: they're taking a fraction of the dose used in the trials that produced positive results.
Always read the EPA and DHA numbers specifically, rather than the total fish oil weight. For general health, a useful daily target is 1,000 to 2,000mg of combined EPA and DHA. In practice that means 2 to 4 standard softgels, not one.
What it's actually good for
The clearest, most replicated effect is on triglycerides. Prescription-strength omega-3 at 4g a day, EPA-heavy, lowers them by 20 to 30 percent in people whose levels are high. That is why it holds an FDA approval at that dose.
General cardiovascular risk is murkier. Large trials like VITAL found little benefit on hard endpoints at standard doses, while EPA-specific trials at higher doses, REDUCE-IT among them, did find meaningful event reduction.
Outside cardiovascular health, there's reasonable evidence for reducing joint stiffness in rheumatoid arthritis and for supporting mood alongside standard treatment for depression, though it isn't a replacement for first-line care.
Picking a product that isn't rancid
Fish oil oxidizes easily, and oxidized oil doesn't just lose effectiveness, it may do more harm than good. Smell the capsule if you can, it should smell mild, not sharply fishy or metallic. Buy from brands that publish third-party oxidation testing (TOTOX values), store the bottle in the fridge, and don't buy more than a 2-3 month supply at a time.
If fish oil causes burping or reflux, taking it with food, splitting the dose, or switching to a triglyceride-form (rather than ethyl ester) product usually helps, triglyceride form also absorbs somewhat better.