Fish liver oil, particularly cod liver oil, contains high levels of omega-3 fatty acids (EPA and DHA), as well as vitamins A and D. There is some scientific evidence suggesting that omega-3 fatty acids may benefit patients with congestive heart failure (CHF). Several clinical trials and meta-analyses have shown that supplementation with omega-3 fatty acids can reduce mortality and hospitalization in patients with heart failure, though the effect size is modest. The GISSI-HF trial, a large, well-conducted study, demonstrated a small but statistically significant reduction in mortality and hospital admissions for heart failure patients taking omega-3 supplements.

However, most studies used purified fish oil supplements rather than fish liver oil specifically. Fish liver oil also contains high amounts of vitamin A, which can be toxic in large doses and is not necessary for heart failure treatment. As such, while the omega-3 content provides a scientific rationale for its use, fish liver oil itself is not the preferred preparation due to potential vitamin toxicity. Guidelines from major heart associations recommend omega-3 fatty acids for some heart failure patients, but do not specifically recommend fish liver oil.

In summary, there is modest scientific support for the use of omega-3 fatty acids in heart failure, but fish liver oil itself is not widely recommended due to safety concerns and has not been the focus of major clinical trials for this indication.