Niagen (nicotinamide riboside) is generally well-tolerated, but minor reported side effects include mild gastrointestinal upset, nausea, indigestion, headaches, or fatigue in rare cases. These symptoms are typically transient and subside as the body adjusts to the supplement. Users are advised to take the capsule with food or lower the dosage temporarily if mild stomach sensitivity occurs, and to consult a physician if any persistent adverse reactions manifest.