There is a version of a skincare routine that looks impressive on a shelf and performs badly on a face. It usually has four actives in it.
The diagnostic problem
If you use a retinoid, an acid, a vitamin C and a niacinamide serum in the same week and your skin reacts, you have no way to identify the cause. You will either stop everything and lose whatever was working, or add a soothing product on top and hope. Both outcomes waste months.
The tolerance problem
Actives that are individually fine can be cumulatively irritating. Each one asks something of the barrier (a lower pH, faster turnover, a temporary increase in permeability) and the barrier has a finite budget. Spending all of it at once is how people end up sensitised to products they used comfortably for years.
How to introduce one properly
Add a single active on alternate evenings. Keep the cleanser, moisturiser and sunscreen unchanged. Give it two full weeks before deciding, most actives take longer than that to show a benefit, but irritation shows up early, so two weeks is enough to catch a problem.
What to do if it stings
Stop that product. Do not add something to calm it while continuing to use it. Return to the four basic steps for two weeks and then try a lower concentration or a different vehicle.