Vitamin C Serum: What It Does and How to Use It

Short answer: Vitamin C serum is an antioxidant treatment, usually made with L-ascorbic acid, that you apply in the morning before sunscreen. S03 S07 Small trials suggest it can make skin look smoother and help with uneven tone, but reviewers say the clinical evidence is limited and more studies are needed. S02 S04 Look for a strength of 10% to 20% in dark or tinted packaging, and expect to wait about three months to see a difference. S07

Vitamin C is one of the most popular skincare ingredients, and one of the trickiest to formulate. Here's what the research shows, what to look for on the label, and how to fit it into your routine.

What vitamin C serum is

Vitamin C is the most plentiful antioxidant in human skin. S03 In skincare, it's usually listed as ascorbic acid or L-ascorbic acid. S07 Cleveland Clinic notes it can come in serums, creams and lotions. S07

Researchers credit topical vitamin C with four main roles: as an antioxidant, for sun-damage protection, for anti-aging (through collagen), and for lightening pigmentation. S02 Vitamin C helps the enzymes that stabilize and cross-link collagen, the protein that keeps skin firm. S03

Your skin depends on vitamin C. The deficiency disease scurvy causes fragile skin and poor wound healing, and a 2017 review called vitamin C's benefit for wound healing its most dramatic and reproducible effect on skin. S01

Eating vitamin C vs. applying it: A 2017 review found that once blood levels of vitamin C are saturated, applying it to skin doesn't appear to raise skin levels further. S01 Topical vitamin C seems to add the most when blood levels are low, and its effect depends on the formula. S01

What the research shows

Uneven tone and dark spots

A 2023 systematic review of seven randomized trials (139 volunteers) found that skin treated with vitamin C showed significant lightening on objective measures of pigmentation. S04 The reviewers said long-term use may be needed for noticeable changes. S04

A separate analysis of 31 small controlled studies (741 healthy volunteers) found that vitamin C reduced darkening caused by simulated daylight UV, more so at higher doses, with a strong effect at 10%. S06 It did not speed fading once skin had already darkened, which suggests vitamin C helps prevent new pigmentation more than it fades existing spots. S06 All of that study's authors worked for L'Oréal. S06

Fine lines and texture

In the 2023 review, trials that measured skin texture found treated skin was smoother and less wrinkled, supported by biopsy data. S04 Hydration improved equally with vitamin C and placebo. S04

A second 2023 review of wrinkle studies (2015 to 2022) found seven studies, four of them high-quality. But every study combined vitamin C with other ingredients or treatments, so the effect of vitamin C alone couldn't be pinned down. S05

Sun protection

Vitamin C does not replace sunscreen. S07 Lab studies found that 10% topical vitamin C reduced UVB-related redness by 52% and sunburn cell formation by 40% to 60%. S03 A Cleveland Clinic dermatologist says vitamin C and sunscreen "work in synergy." S07

The main studies at a glance

How strong is the evidence?

Limited. Reviewers in 2017 wrote that clinical studies on topical vitamin C "remain limited." S02 The 2023 reviews included only seven trials each, and the melasma and photoaging review covered just 139 people. S04 S05 Neither review settled the ideal concentration. S04 Many studies paired vitamin C with other ingredients or lasers, which makes it hard to know what vitamin C did on its own. S04 S05

Which form and strength to look for

What about derivatives? Brands often use more stable forms of vitamin C. A 2017 review notes that phosphate forms are more stable but poorly absorbed through skin, and that it's unclear whether palmitate forms convert efficiently to active vitamin C. S01

Vitamin C + E + ferulic acid: A dermatology review notes that vitamin C works best alongside vitamin E, and that 15% vitamin C with 1% vitamin E and 0.5% ferulic acid increased vitamin C's effect eight-fold. S03

To find vitamin C on an ingredient list, our guide to reading a skincare label explains where to look.

How to use vitamin C serum

1. Cleanse first. Apply vitamin C after cleansing, and after toner if you use one. S07
2. Use it in the morning. It's safe at night too, but Cleveland Clinic's dermatologist recommends mornings because it backs up your sunscreen. S07
3. Follow with sunscreen. S07
4. Be patient. Expect about three months before you see results. Taking a bare-faced photo once a month can help you track changes. S07
5. Take care around the eyes. S03

How often? Cleveland Clinic says you can use vitamin C on your skin every day. S07 A dermatology review notes that after vitamin C reaches its peak level in skin, its half-life there is about four days, and that a steady reservoir in the skin, built through regular use, matters for sun protection. S03

Vitamin C can be used alongside sunscreens, tretinoin, other antioxidants and alpha hydroxy acids like glycolic acid. S03 If you're building a routine, our simple skincare routine by skin type shows where a treatment serum fits. For a gentler treatment step, see our niacinamide guide.

Side effects and patch testing

Most people don't have side effects, but rarely vitamin C serum causes dryness, itching or redness. S07 A dermatology review adds stinging, and says these milder reactions can usually be treated with a moisturizer. It also notes that once applied, vitamin C can't be fully washed or wiped off the skin, and that hives and a skin reaction called erythema multiforme have been documented. S03

Oxidized vitamin C can leave a yellowish tint on skin, lighten hair and stain clothes. S03 Vitamin C serum is almost colorless when fresh and turns yellow as it oxidizes on exposure to light. S03

If your skin is sensitive, Cleveland Clinic suggests starting with a lower concentration. S07 Before adding any new serum, our guide on how to patch test skincare walks you through the steps.

Frequently asked questions

What does vitamin C serum do for your skin?

Vitamin C is an antioxidant. Reviews of small trials found topical vitamin C made skin look smoother and less wrinkled and lightened pigmentation on objective measures. The authors said more studies are needed.

What percentage of vitamin C serum is best?

A Cleveland Clinic dermatologist says under 10% L-ascorbic acid won't deliver the benefits you're looking for, and above 20% risks irritating your skin. A dermatology review notes that effectiveness rises with concentration only up to about 20%. If your skin is sensitive, start low.

Should I use vitamin C serum in the morning or at night?

Either is safe, but Cleveland Clinic's dermatologist recommends the morning, after cleansing and toner, because vitamin C works alongside sunscreen. It does not replace sunscreen.

How long does vitamin C serum take to work?

Cleveland Clinic's dermatologist says it takes about three months to really see the effects. A systematic review also noted that long-term use may be needed for noticeable changes.

Why did my vitamin C serum turn yellow?

Vitamin C is unstable and oxidizes when exposed to light, which gives it a yellow color. Choose a serum in dark or tinted packaging. Oxidized vitamin C can also leave a yellowish tint on skin and stain clothes.

Can I use vitamin C serum with other products?

A dermatology review says topical vitamin C can be used with sunscreens, tretinoin, other antioxidants and alpha hydroxy acids such as glycolic acid. Combining vitamin C with vitamin E and ferulic acid was found to boost its protective effect.