According to the search results, hyaluronic acid and niacinamide can be used together in a skincare routine. They are both water-based treatments that complement each others hydrating and youth-boosting effects. When layering these two ingredients, it is recommended to apply hyaluronic acid first to hydrate the skin cells, followed by niacinamide to help regulate sebum production and minimize pores. However, it is also noted that there is no right or wrong way to layer these two ingredients, and it ultimately depends on personal preference. It is also suggested to start with a low concentration of each ingredient and gradually increase as tolerated over time, especially for those with sensitive skin. Overall, hyaluronic acid and niacinamide are safe to use together and can provide multiple benefits for the skin.