If you pick one, azelaic acid has the more direct evidence for fading the look of dark marks, mostly at 15 to 20% strengths, while vitamin C's trials are fewer and its results depend heavily on the form and formula. You do not have to choose: vitamin C fits the morning under sunscreen and azelaic acid fits the evening. For age spots after 40, the more useful test of any product is whether it lightens the spot more than the skin around it, because that contrast is what you actually see.
The routine for using both is set out below. If your concern is the look of sun-related age spots, our Cellular Clarity Serum was built around that spot-to-skin contrast, and how it fits is covered at the end.
What is each ingredient actually studied for?
Azelaic acid is studied mostly at prescription-level strengths. A 2025 review reports that 15 to 20% formulations produced meaningful improvements across its studied uses, with mild, transient local irritation, and that performance depends strongly on the vehicle (Petrovici et al., 2025). Many of those uses are medical diagnoses, so those results belong to those studies, not to a cosmetic serum. A 2023 review notes that products with cosmetic and with drug status both exist at 5 to 20% (Sauer et al., 2023), so it is worth checking which one you have.
Vitamin C has a smaller body of trials. A 2023 systematic review found seven randomised trials with 139 volunteers in total: treated skin looked smoother and less wrinkled, and objective measures showed lightening, but the authors say long-term use may be needed and more studies are required (Correia and Magina, 2023). A 2026 review adds that results depend on the vitamin C derivative, the vehicle, the pH and how it is stabilised (Boghosian et al., 2026). In other words, "vitamin C" on a label is not one thing.
How do they compare side by side?
| Azelaic acid | Vitamin C | |
|---|---|---|
| Studied for | Uneven pigment, including marks left after breakouts, mostly at 15 to 20% | Signs of sun damage and uneven pigment |
| Timescale in studies | Pigment changes measured at 8 to 12 weeks in a 2024 trial | Reviews suggest long-term use; seven trials, 139 people |
| Irritation | Mild, transient local irritation reported | Depends on form; some derivatives are chosen for sensitive skin |
| Morning or evening | Either; evening keeps it apart from vitamin C | Morning, under sunscreen |
| Prescription? | Cosmetic and drug-status products both exist | Cosmetic |
Both columns come with an honest caveat. In a 2024 placebo-controlled trial, 72 people with marks left after breakouts applied a 15% azelaic acid gel or a plain gel twice a day for 12 weeks, and 60 finished. Brown marks faded from baseline in both groups at 8 and 12 weeks, and the measured drop in melanin with azelaic acid was significant only at 12 weeks (Shucheng et al., 2024). In the vitamin C review, hydration improved equally on vitamin C and placebo sites. Some of what people credit to an active is time, sunscreen and moisturiser.
Can you use azelaic acid and vitamin C together?
Yes, and the simplest way is to give each its own time of day. Vitamin C goes in the morning under sunscreen, where its antioxidant role makes most sense: in a laboratory study, adding ferulic acid to a 15% vitamin C and 1% vitamin E solution made it more stable and roughly doubled its protection of skin against simulated sunlight, from about 4-fold to about 8-fold (Lin et al., 2005). That was measured in the lab, and it is no substitute for sunscreen. Azelaic acid then goes in the evening.
- Morning: cleanse, vitamin C serum, moisturiser, broad-spectrum SPF 30 or higher.
- Evening: cleanse, azelaic acid, moisturiser.
- Introduce one at a time. Patch test each new product on the inner arm and wait 48 hours.
- Alternate nights if you also use acids, exfoliants or retinoids.
Sunscreen does more work in this routine than either active, and not every SPF covers every kind of light. Our piece on why dark spots can darken despite SPF explains the gap, and how to layer serums morning and evening covers the order of steps.
Which one fits which kind of spot?
Match the product to the spot, and get the spot identified first. Marks left after breakouts are where much of the azelaic acid research sits. Flat brown sun spots on the face and hands, the classic age spot, are where the contrast between spot and surrounding skin matters most. Patchy, symmetrical pigment across the cheeks or forehead is a question for a dermatologist, not a shopping one. Our earlier pieces on what the azelaic acid research shows and vitamin C for uneven tone after 35 go deeper on each ingredient.
When should a dark spot be seen by a dermatologist?
Before you treat it, if it is new, changing in size, shape or colour, bleeding, itching, or simply looks different from your other spots. A cosmetic serum is for the appearance of a spot you already know is harmless. Patchy pigment over large areas of the face should also be looked at first.
Where does Cellular Clarity fit?
Cellular Clarity Serum is made for the look of age spots, and specifically for the gap between a spot and the skin beside it. Its lead ingredient is Delentigo™ at 4%, the concentration its maker tested, alongside MossCellTec™ No. 1.
In the ingredient maker's 8-week placebo-controlled study, a cream with 4% Delentigo™ was applied twice a day to age spots on the hands of 12 women aged 40 to 66. Before treatment, the spots had a melanin reading 26% higher than the skin around them. After 8 weeks that gap was 16.5%. The surrounding skin lightened too, but the spots lightened more. In a shorter 4-week study of 10 women aged 47 to 77, treated spots read 6.2% lower than placebo-treated spots, with no difference in normal skin. These studies were run by the ingredient maker on a test cream, not on our finished serum, and they are not peer-reviewed.
A vitamin C ester and Azeloglicina®, an azelaic acid derivative, are in the base below the levels their own studies used, and we make no claim for either. Apply it in the morning, under sunscreen. It contains soy isoflavones and 0.34% alcohol, so patch test for 48 hours before first use. The full ingredient file is on the Cellular Clarity science page.
Frequently asked questions
Is azelaic acid or vitamin C better for age spots?
Neither has strong trial evidence specific to age spots on the face. Azelaic acid's research is mostly at 15 to 20% for other pigment concerns, and vitamin C's rests on seven small trials, so the formula and your sunscreen habit matter as much as the ingredient name.
Can I use azelaic acid and vitamin C on the same day?
Yes. The simplest routine is vitamin C in the morning under sunscreen and azelaic acid in the evening, introducing one product at a time.
How long before I see a difference in the look of dark spots?
The studies cited here measured at four, eight and twelve weeks. Give any product at least that long, with daily sunscreen, before you judge it.
Is cosmetic azelaic acid the same as prescription azelaic acid?
Not necessarily. Products with cosmetic and with drug status both exist between 5 and 20%, and most of the published research uses the higher strengths.
Do I still need sunscreen with these ingredients?
Yes. Broad-spectrum SPF 30 or higher every morning is the base of any routine for the look of dark spots.
Written by the VyVa Editorial Team.
This article is general information about cosmetic skincare and the appearance of skin. It is not medical advice and is not intended to diagnose, treat, cure or prevent any disease. Results vary between individuals. Patch test any new product on the inner arm and wait 48 hours before first use.
