Its product page advises against using it on sensitive or broken skin. If your skin reacts easily, the Alltimate Niacinamide 10% Serum is the gentler choice, as it is hypoallergenic and fragrance-free. Patch test either one first.

Yes, in most routines. Use the antioxidant serum in the morning and retinol at night. If you are new to both, start one at a time.

Many routines combine them. You can apply the Multi Vita serum in the morning and the Niacinamide 10% Serum at night, or layer them, starting slowly to see how your skin responds.

Yes. It is a sunscreen with SPF50 PA++++, so it can be your final morning step. It does not replace the antioxidant serum, which goes on underneath it.