Fish oil is a naturally-occurring fatty acid your body needs to function optimally. This essential fatty acid, also called omega-3, may also help lower the levels of triglycerides in your blood. If you are an adult, do not take more than two to three 1,200 mg doses of fish oil per day unless otherwise advised by a health care professional, the University of Maryland Medical Center reports. Additionally, you should be aware of the side effects of fish oil before taking this omega-3 fatty acid supplement.

Belching or Bloating

Fish oil supplements may cause excess gas to accumulate in your digestive tract. Gassiness may cause you to feel bloated or you may notice that your clothing fits tighter than usual around your waist. You may also expel excess gas from your body by belching or flatulence. These side effects of fish oil may be initially embarrassing but may diminish with continued use of this supplement.

Diarrhea

Stomach irritation caused by fish oil supplements may result in diarrhea. You may experience frequently loose bowel movements that are accompanied by abdominal cramping or fullness. Chronic diarrhea may cause your body to lose too much fluid, leading to symptoms of dehydration. If you experience persistent episodes of diarrhea while taking fish oil supplements, consult your physician for further evaluation and care.

Bad Breath

Treatment with fish oil supplements may cause your breath to give off a foul, fish-like odor. You may also develop an unpleasant taste in your mouth, Drugs.com warns, which may exacerbate upset stomach side effects.

Unusual Bleeding or Bruising

Taking high doses of fish oil may cause you to bleed more easily following minor injuries or surgical procedures. Consult your doctor if you develop any usual bruises or if you experience nosebleeds while taking this type of supplement.