Side effects of fish oil include unusual weakness or tiredness, bleeding gums, coughing and nosebleeds. Other possible side effects include irregular or fast heartbeat, prolonged bleeding from wounds, dizziness and headache. Some people may experience red or black, tarry stools, red or dark brown urine, or paralysis, according to Drugs.com.
Additional possible side effects of fish oil include tightness in the chest, hives, skin rash, itching, and puffiness or swelling around the eyes, lips, tongue or face. Some women may experience increased menstrual flow or vaginal bleeding. If side effects occur, a physician should be contacted immediately, notes Drugs.com.
Less common side effects of fish oil include heartburn, indigestion, diarrhea and gas. Other possible side effects include constipation, acid or sour stomach, bloating and belching. Some people may experience a loss of appetite, a change in taste, vomiting or fullness, states Drugs.com.
Sometimes the side effects of fish oil subside after the body gets used to the supplement. Not all side effects require medical attention, but checking with a health care professional is recommended to be safe. Some medications may interact with fish oil, so it is important to inform a physician or pharmacist about any supplements or medications being taken before taking fish oil, notes Drugs.com.