Fish oil contains beneficial fatty acids called omega-3s and has been found to be effective in reducing high triglycerides. Available in supplements as well as from fatty fish, such as mackerel, sturgeon, salmon, anchovies and sardines, fish oil may also be beneficial in reducing the risk of cardiovascular disease. Many people take multivitamin supplements with their fish oil or they may take fish oil separately.
Fish oil and multivitamins have some similar side effects, according to MedlinePlus. Both can cause symptoms of an upset stomach such as belching, heartburn or nausea. If you take them together, that could make those symptoms worse. MedlinePlus notes that taking fish oil with a meal or freezing the supplements can reduce gastrointestinal side effects. You might also try switching to a different brand of multivitamin or fish oil to decrease gastrointestinal side effects or take them at different times of the day. If you aren't having any problems with side effects you can take them together.