Taking more than 3 grams of omega-3 can cause bleeding complications, loose stools, upset stomach and low blood pressure. Consuming omega-3 is considered safe when taken in the recommended dosage for up to two to three years, according to Mayo Clinic.
Certain fish, such as salmon and sardines, are the primary sources of omega-3. Although it is considered one of the healthiest foods, fish-derived omega-3 may contain mercury and should be taken with caution by children and pregnant women, states Mayo Clinic. Individuals who have an allergy to fish but are not allergic to nuts, plants or seeds may take omega-3 derived from flaxseed, walnuts and Brussels sprouts to avoid allergic reactions, recommends The World's Healthiest Foods.