Cod liver oil can act like a blood thinner. Before starting a cod liver oil regimen, definitely talk to your doctor if you’re already taking blood thinners or high blood pressure medicine or are pregnant or asthmatic. Other minor side effects can be fishy belching (not harmful to your health), nausea, heartburn, bloody stool, and low blood pressure. Also, consider monitoring your vitamin A and D levels while taking cod liver oil to avoid toxicity.