Increase the risk of bleeding: As fish oils lessen the adhesiveness of blood platelets, they decrease the blood clot formation which leads to increased bleeding. This side effect may occur in low doses, but in high doses, the effects are highly risk such as hemorrhagic stroke.  

Gastrointestinal upset: It is one of the well known causes. It leads to variety of digestive problems such as bloating, flatulence, acid reflux, sickness, and diarrhea. This can be controlled by taking fish oil supplements with diet. People with history of bleeding or blood thinner people are not recommended to take this fish oil supplements. 

Elevation in LDL cholesterol: The higher doses of fish oil supplements may increase the LDL cholesterol level up to 10 percent. A recent study has proved that regular intake of fish oil supplements can lead to the reduction of LDL cholesterol level. 

Possibility for contaminants: Some of the varieties of fish oil supplements are known to be contaminated with toxicants such as dioxins, heavy metals, and PCB’s.  

Other side effects of fish oil supplements include lower blood pressure, fishy taste in the mouth, nausea and burping. To prevent all these side effects, you can start with low dose.