Cod liver oil

As a supplement, cod liver oil is a mixed blessing. On the plus side, it is loaded with omega-3 fatty acid, like most other fish oils. It also has a substantial amount of Vitamin D - enough to cure rickets, at least.
On the other hand, it has an enormous amount of Vitamin A. Which not only weakens its value as a source of Vitamin D, but also means it is easier to take toxic amounts than with other supplements.