Ask a dozen dermatologists about their favorite skin care ingredient, and you’ll likely get a dozen love letters about vitamin C. “It’s one of the few ingredients that both protects and repairs skin, but is also tolerated by a vast majority of patients,” says Dr. Titilola Sode, a board-certified dermatologist in Dallas. But not all vitamin C serums are equally effective, and the wrong formula, packaging or even application can render it useless. So before you stock up on serums, make sure you’ve got these seven vitamin C facts straight.

4)It needs to be closest to your skin.

“Vitamin C needs to be the first thing that goes on your face in the morning, otherwise it’s not going to absorb and protect your skin,” says Dr. Sode. So all those toners and essences you’ve been massaging on before your vitamin C? Move ’em to the P.M.

5)It actively fights against dark spots.

Unlike most skin brighteners that work to fix damage after the fact, vitamin C is consistently working to prevent hyperpigmentation in the first place. “Vitamin C interrupts tyrosinase—an enzyme that’s responsible for creating pigment in skin cells—while preventing the pigment you already have from getting darker,” says Dr. Sode. It’s smart like that.