Read more: 4 Benefits of Fish Oil Supplements — and 3 Risks You Should Know AboutConsider Fish Oil BenefitsAccording to Mayo Clinic, omega-3 fatty acids in fish oil can significantly impact blood sugar levels that are too high. Beware Fish Oil Side EffectsMost side effects associated with fish oil are non-serious and temporary. Magnesium Supplements Side EffectsNegative side effects of magnesium supplements can occur when you start using it. Fish Oil and Magnesium InteractionsAs fish oil lowers your triglycerides and begins to clear out your arteries, your blood pressure will decrease, according to Mayo Clinic. For this reason, ask your physician about the possibilities of interactions between fish oil and magnesium supplements before you use them together.