The short answer? Serums go first, no doubt about it. “It’s best to layer lighter products like serums before heavier products like moisturizers,” board-certified dermatologist Hadley King, M.D. tells mbg.
This is because moisturizers contain some occlusive ingredients, which help them trap moisture in the skin but prevent other ingredients from penetrating. So if you layer on a serum over your face cream, the likelihood of your skin absorbing that product and thus reaping all of the benefits is slim.
However, there is one caveat here: “If you want to decrease the potency of a serum—like if it contains potentially irritating ingredients like retinol or salicylic acid, then applying a moisturizer first could help make the serum less potent and less irritating,” King notes.
This technique is often referred to as the “retinol sandwich,” where you layer moisturizer under and over your retinol to buffer the effect. But should you do this with other serum ingredients like hyaluronic acid, niacinamide, and vitamin C? No, unless you think these serums might trigger a negative reaction from the skin (which is pretty unlikely for most people).
In fact, the deeper these ingredients can penetrate, the better. So to sum it all up: Potentially irritating serums can be buffered by using a moisturizer before and after, but otherwise, all serums should be applied first.