The most common side effects of taking NMN supplement UK include nausea, vomiting, stomach upset, diarrhoea, and fatigue. In some cases, users may also experience headaches, dizziness, or difficulty sleeping. It is important to note that these side effects are usually mild and will likely disappear once your body grows accustomed to the supplement. If you experience any severe or persistent side effects when taking NMN supplement UK, it is important to consult your healthcare provider immediately. It is also recommended that you speak with a doctor before beginning any nutritional supplement regimen to ensure it is safe and appropriate for your individual needs.