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.

6)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.

7)It has to have the right packaging to work.

“Biomedically speaking, vitamin C is very finicky,” says Dr. Sode. “Even the best formulas can quickly oxidize and become worthless if exposed to sunlight.” Your vitamin C serum should be in an airless pump container (like Allies of Skin Vitamin C 35% Serum) or housed in a an opaque or tinted glass bottle (like Obagi Professional-C Serum 20%). “I store mine in a dark drawer and try to open and close the bottle quickly to prevent visible light from getting in,” she says.