Cod Liver Oil Safety and Side Effects. Cod liver oil is safe for most people. It can cause side effects including belching, bad breath, heartburn, and nosebleeds. Taking cod liver oil with meals can often decrease these side effects. High doses might keep blood from clotting and can increase the chance of bleeding. Vitamin A and vitamin D levels might also become too high with high doses of cod liver oil. High doses might also cause nausea and loose stools.