Which serum is actually best? The answer left behind by medical research: "Vitamin C" and "Retinol" - Part 1 of the "Choosing the Truly Necessary Serum Based on Medical Research" series

Standing in front of the bathroom mirror, I find myself hesitating while holding a serum.

Vitamin C, retinol, niacinamide, peptides, hyaluronic acid. Recently, there are even products labeled with EGF, exosomes, and PDRN.

“In the end, which one should I use?”

Even in my own work of reading medical papers, I am overwhelmed by the sheer number of ingredients lined up on cosmetic shelves. Prices range from a few thousand to tens of thousands of yen, and while it feels like more expensive products should be more effective, the impression created by advertisements does not necessarily align with medical evidence.

That is why I am starting the “Choosing the Truly Necessary Serum Based on Medical Research” series.

In this first installment, I will cover Vitamin C and Retinol, which have been studied for a long time among the many beauty ingredients available.

By reading this article, you will understand why these two are central to choosing a serum, what you can expect from them, and where their limitations lie.

A serum is not about “how expensive it is,” but “what is inside”

In cosmetics, attractive product names and luxurious containers are easy to notice. However, when considering the effects on the skin, what is more important than the brand name are the following points:

Which active ingredient is formulated, and in what form?

Is the ingredient designed to reach the skin?

Has its effectiveness been confirmed in comparative studies involving humans?

Furthermore, the effectiveness of a serum also changes depending on the concentration, pH, container, stability, and other ingredients included.

In other words, just because a product says “contains Vitamin C” or “contains Retinol” does not mean all products have the same effect.

Nevertheless, when organizing current research, Vitamin C and retinoids emerge as ingredients with a relatively large number of clinical studies regarding fine wrinkles caused by photoaging, uneven skin tone, and skin roughness [1-3].

Vitamin C is responsible for “protecting” and “conditioning”

When exposed to factors such as ultraviolet rays, oxidative stress increases in the skin, and reactions related to changes in skin tissue, including collagen, and pigmentation progress.

Vitamin C has antioxidant properties and is also involved in the reactions necessary for collagen synthesis. It also acts on the melanin production process and may improve uneven skin tone and pigmentation.

A systematic review published in 2023 suggested that topical Vitamin C improves skin unevenness, wrinkles, photoaging, and melasma. On the other hand, because formulations, concentrations, concomitant treatments, and evaluation methods differ from study to study, it is considered that we are not yet at a stage where the optimal concentration or formulation can be definitively determined [1].

What I want to note here is that Vitamin C is not a substitute for sunscreen.

Vitamin C serum does not provide sufficient protection against ultraviolet rays; it serves as a supplement for oxidative stress that sunscreen cannot fully prevent. Therefore, even when used in the morning, you must apply sunscreen afterward.

Also, Vitamin C is not an ingredient that erases dark spots in a few days of application. Research has pointed out that visible changes may require long-term use [1].

Retinol works on the skin's structure

Retinol is a type of retinoid, which is a member of the vitamin A family.

Once applied to the skin, retinol is converted through retinal and finally into retinoic acid to take effect. It regulates reactions related to epidermal cell turnover and dermal collagen, and is expected to improve fine lines, skin roughness, and uneven skin tone.

However, retinol in cosmetics is not the same as tretinoin used as a pharmaceutical.

Tretinoin is retinoic acid itself and does not require conversion, making it more potent, but it is also characterized by a tendency to cause redness, dryness, peeling, and irritation.

A 2024 systematic review examined 25 studies comparing tretinoin with other topical treatments. While tretinoin is the standard ingredient for treating photoaging, the review concluded that due to low quality and bias in comparative studies, there is insufficient evidence to definitively state that less irritating alternative ingredients outperform tretinoin [2].

Furthermore, a 2025 network meta-analysis examined data from 23 RCTs with a total of 3,905 participants, indicating that topical retinol, tretinoin, and isotretinoin improve fine lines, and that retinol and tretinoin may also be effective for hyperpigmentation [3].

However, this research has limitations, including a lack of racial diversity, variations in products and evaluation methods, and the potential for commercial bias. You cannot judge that the same product is optimal for everyone just by looking at ingredient rankings [3].

Vitamin C or Retinol: Which is better in the end?

Neither is completely superior to the other. Their roles are slightly different.

Concerns suited for Vitamin C

Concerned about dullness or uneven skin tone
Want to suppress oxidative damage caused by UV rays
Want to incorporate it into morning skincare
Worried about the irritation of retinol

Concerns suited for Retinol

Concerned about fine lines
Want to improve skin roughness or lack of firmness
Want to continue anti-aging care over several months
Can adjust usage frequency according to redness or dryness

In practice, using Vitamin C in the morning and Retinol at night is a rational approach.

However, you do not need to start both at the same time from the beginning. It is easier to identify the cause if irritation occurs if you introduce one first, check your skin's reaction, and then add the other.

The first thing you should buy is not a serum

I have introduced Vitamin C and Retinol so far, but neither of them should be the first priority in anti-aging care.

The first things you need are sunscreen and moisturizer.

Chronic UV exposure is a major factor in skin photoaging. Adding only expensive serums without using sunscreen is like continuing to pour water into a container with holes in it.

First, continue using sunscreen and moisturizer.

On top of that,

Vitamin C in the morning
and low-concentration retinol at night

should be added little by little, to the extent that they do not cause irritation.

What remains after reading through medical literature is not some rare new ingredient, but this kind of unassuming yet easy-to-sustain combination.

However, products labeled 'Vitamin C' include both pure vitamin C and various vitamin C derivatives. Retinoids also include retinol, retinal, and tretinoin, and their efficacy and irritation levels are not the same.

Next time, we will compare vitamin C and retinol head-on.

For dark spots, pores, and wrinkles, which one should you ultimately choose?

We will organize this more specifically by concern. Click here for Part 2☟.

References

[1] Correia G, Magina S. Efficacy of topical vitamin C in melasma and photoaging: A systematic review. Journal of Cosmetic Dermatology. 2023;22(7):1938-1945. doi:10.1111/jocd.15748.

[2] Siddiqui Z, Zufall A, Nash M, Rao D, Hirani R, Russo M. Comparing tretinoin to other topical therapies in the treatment of skin photoaging: A systematic review. American Journal of Clinical Dermatology. 2024;25(6):873-890. doi:10.1007/s40257-024-00893-w.

[3] Lin L, Chen X, Liu C, Wang Q, Lian W, Xu X, et al. Comparative efficacy of topical interventions for facial photoaging: A network meta-analysis. Scientific Reports. 2025;15(1):26889. doi:10.1038/s41598-025-12597-0.