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.

1. There are a zillion forms of vitamin C.

Most vitamin C derivatives are thought to have the same effect (they can tone down redness, hyperpigmentation and signs of aging), but some forms have added benefits and are better studied, says Dr. Sode. The most popular and effective:

L-ascorbic acid—i.e., pure vitamin C—has the most research behind it and tends to be the strongest (but most sensitizing). Try: SkinCeuticals C E Ferulic Acid

Tetrahexyldecyl ascorbate is fat soluble, meaning it can penetrate deeply and effectively into your skin. Try: Revision Skincare® C+ Correcting Complex 30%

Magnesium ascorbyl phosphate can increase hydration levels in dry skin and prevent water loss. Try: 100% Pure Vitamin C Serum

Sodium ascorbyl phosphate tends to have a neutral pH (making it gentle on sensitive skin), along with acne-inhibiting effects. Try: IMAGE Skincare VITAL C Hydrating Antioxidant A C E Serum

2. It’s one of the few ingredients that can actually smooth fine lines.

A lot of products boast about their anti-aging powers, but unless they contain ingredients known to trigger the production of collagen (a very short list that includes retinoids, growth factors and—ding, ding—vitamin C), they won’t make a huge dent in slowing down your skin’s natural aging process.

3. It’s shockingly good at protecting your face.

“On a cellular level, vitamin C is a great antioxidant—it hunts down and destroys the free radicals in your skin, which break down the DNA of everything that keeps you youthful,” says Dr. Sode. Free radicals can’t be avoided—they come from sunlight, pollution, chemicals and even our own bodies—but they can be neutralized by vitamin C.

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.

6. 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.

7. It can actually work pretty quickly.

Though results depend on your skin type, the formula you’re using and how often you apply it, research suggests that vitamin C serums of 10 to 20 percent can start soothing redness after just a few weeks and brightening hyperpigmentation within three months. “Just remember that vitamin C isn’t a one-time thing—you need to use it long-term for the full effects,” says Dr. Sode.