In 1933, at ETH Zurich, the chemist Tadeus Reichstein and his colleague Andreas Gruessner worked out how to make ascorbic acid from glucose. Until then, vitamin C had to be extracted from sources such as paprika. The method is still called the Reichstein process. The drug company Hoffmann-La Roche bought the patent rights and, in 1934, became the first to mass-produce synthetic vitamin C, sold as Redoxon. That company has no relation to La Roche-Posay, despite the name. What matters is that a vitamin nobody could easily get became something you can buy in a dropper bottle.

La Roche-Posay vitamin C is a good place to see what a pharmacy brand does with it. The modern brand dates to 1975, when the pharmacist René Levayer founded it, and L'Oréal bought it in 1989. The serum sits in that dermatologist-facing tradition. It is, per the label, 12% pure vitamin C with salicylic acid, hyaluronic acid, neurosensine and thermal spring water, and it was quietly renamed from Pure Vitamin C10 to Vitamin C12. That rename is a useful lesson: read the percentage on the label, not just the name.

The trick is to treat the serum as a morning step. The brand says to put three or four drops on face and neck after cleansing, keep it away from the eye contour, and follow with a moisturizer and SPF. The Pure 12% Vitamin C Serum for Wrinkle Reduction and Radiance is the flagship, and the Vitamin C12 Oil Control Face Serum is the version for shinier skin. The Pure Vitamin C Anti-Wrinkle Firming Eye Cream handles the area the serum should not touch.

Around those, the brand has supporting serums. Mela B3 Serum Dark Spot Corrector and the Mela B3 Double Dose Day & Night Intensive Treatment go after marks, Retinol B3 is the evening serum for fine lines, Hyalu B5 is the plain hyaluronic acid one, and Glycolic B5 is the exfoliating option.

Leave a new vitamin C serum on one cheek for a few mornings before it goes on your whole face. It is the simplest patch test there is, and vitamin C can sting skin that is already irritated.

There are plenty of formats. Sunday Riley's C.E.O. 15% Vitamin C Brightening Serum is the brightening classic of the group. The INKEY List's 15% Vitamin C & EGF Brightening Serum is the plain, no-fuss one. Medik8's C-Tetra Advanced 20% Stabilized Vitamin C Gel-Serum is the one for people who already use vitamin C and want a higher number. Drunk Elephant's C-Luma Hydrabright is a gentle hydrating take, and Glow Recipe's Guava Vitamin C Dark Spot Brightening Treatment Serum is aimed at marks.

Paula's Choice's 25% Vitamin C + Glutathione Clinical Serum is the strongest number on the page, so I would approach it with care. Dermalogica's BioLumin-C is the stable vitamin C one, and Kiehl's Clearly Corrective Dark Spot Correcting Serum with Vitamin C is the familiar apothecary choice.

A higher number is not automatically better. The percentage tells you how much vitamin C is in the bottle, not how well your skin will tolerate it, and plenty of people do better with a lower one every day than a stronger one every other day.

What I would skip: a new vitamin C serum on the same morning as a strong exfoliating acid. Give your skin one job at a time.

Take a photo of the spot you care about, in daylight, before you start. Dark marks fade slowly, and without a photo you will argue with your own memory.

Vitamin C is one route to an even tone. The serums here are the others. Drunk Elephant's B-Goldi Bright Illuminating Drops carry 5% niacinamide, and Glow Recipe's Watermelon Glow Niacinamide Hue Drops add a little color. Facile's Brighten 5% Niacinamide Serum is aimed at dark spots and uneven texture. Dr. Idriss's Major Fade Hyper Serum and banu's Brightening Dark Spot Milky Serum are the more targeted ones.

Then the serious shelf. Allies of Skin's Tranexamic & Arbutin Advanced Brightening Serum and its Azelaic & Kojic Advanced Clarifying Serum each pair two brightening ingredients, and Dr. Barbara Sturm's The Better B Niacinamide Serum is the pared-back one.

A pairing I like: vitamin C in the morning, niacinamide at night, which keeps each one where it is easiest to use and stops the routine from becoming a chemistry exam.

What I would skip: adding three brightening serums at once. You will not know which one helped, and if your skin reacts you will not know which one did it.

Sunscreen is the partner for any vitamin C serum, and the best one is the one you will actually put on. The American Academy of Dermatology advises broad-spectrum, water-resistant SPF 30 or higher, about a teaspoon for the face, reapplied roughly every two hours outdoors.

La Roche-Posay's own Anthelios Ultra Light Fluid Face Sunscreen SPF 60 is the obvious match, and the brand's sun range dates back to 1987. Ultra Violette's Unblock Screen SPF 50 is labeled non-acnegenic, which matters for anyone prone to breakouts. Supergoop's Triple Prep Weightless Multitasking Moisturizer SPF 40 is the light one, its Superscreen Rich Hydrating Cream SPF 40 is the richer one, and The Ordinary's UV Filters SPF 45 Serum Sunscreen is the lightest of the lot.

Shiseido's Urban Environment Vita-Clear SPF 42 actually has vitamin C in it, which is a nice extra rather than a replacement for your serum. Cell Fusion C's Laser UV Defense SPF 50 and Vacation's Classic Whip SPF 50 are the two SPF 50 options, so apply them as generously as the advice above.

What I would skip: trusting the vitamin C serum to do the sun's work. An antioxidant is a supporting act, and sunscreen is the lead.

For most people, my pick is simple: a vitamin C serum you will use every morning, then moisturizer, then SPF. The La Roche-Posay bottle is a sensible first one, and if it turns slightly yellow over time, the brand says it is still effective.

Add a dark spot serum at night only if you need one, and do not chase every new number. Reichstein wanted a reliable source of a vitamin people had once pulled from paprika. You want a reliable habit, which is about as dull and as useful as chemistry gets. If you want to keep browsing, I also put together Grande Lash Serum and Other Lash Serums Worth Knowing and Ciele Skin Tint.

If you would rather get your SPF and a little color in one step, I covered the La Roche-Posay tinted sunscreen fluids too.