I'm an Aesthetician—Believe Me When I Say This Underrated Ingredient Will Massively Advance Your Skincare Routine

There are loads of buzzy new skincare ingredients out there, but sometimes it's safer to stick to the ones that have stood the test of time. Take vitamin C, for example. Once you've found the best vitamin C serum for your skin type and start to use it consistently, you'll see results for skin tone, laxity, dullness and pigmentation that few other ingredients can deliver.

Of course, not all vitamin C serums are created equally. Since vitamin C is a notoriously unstable ingredient, some forms may not be effective or may even cause irritation to skin. Ask any skincare expert or professional and they will tell you that the best and most reliable form of vitamin C is L-ascorbic acid. Why? Because it's the purest form of vitamin C, with extensive proven skin benefits and plenty of data and research to back its efficacy. If you want to firm, brighten, address signs of skin ageing or simply amp up the glow, L-ascorbic acid is guaranteed to deliver results—and you'll find it in a whole bunch of vitamin C products.

To help you find the best L-ascorbic acid serum for your skin type or concerns, I've compiled a list of the ones I love the most. It took a while, mind you. I took into account everything from how they play with other products to whether they triggered sensitivity and, of course, how efficiently they delivered results.

Here's a quick brush up on the ingredient and its benefits, followed by my shortlist of the best L-ascorbic acid serums on the market.

What Is L-Ascorbic Acid?

L-ascorbic acid is the purest form of vitamin C and one of the most common form of vitamin C found in skincare products. It's a very stable, active and quick-to-absorb type of vitamin C, which makes it the most effective form for topical skin treatments.

What Are the Benefits of L-Ascorbic Acid?

"L-ascorbic acid is used in skincare products for its antioxidant properties," says dermatologist Sonia Khorana. "It can help reduce signs of ageing, brighten the skin and even out skin tone." In particular, mature skin showing common signs of ageing like wrinkles, loose skin and sun damage, and acne-prone skin with scarring and hyperpigmentation will significantly benefit from using L-ascorbic acid.

Most people can benefit from using L-ascorbic acid in their skincare routine thanks to its antioxidant properties. As an antioxidant it protects skin from environmental stressors like pollution and sunlight.

Differences Between Ascorbic Acid and L-Ascorbic Acid

Take a look at the ingredients lists on your vitamin C serums and you'll notice they feature either L-ascorbic acid or ascorbic acid. "The two names are used interchangeably but the main thing is that the 'L' indicates that it's the most natural form of the vitamin C," explains Khorana. "Ascorbic acid is a general term that refers to the vitamin C molecule. However, it can exist in two forms: L-ascorbic acid and D-ascorbic acid. In skincare, it’s almost always L-ascorbic acid because the D form is not biologically active and doesn't provide the same benefits as L-ascorbic acid."

Ascorbic acid is often used to describe all forms of vitamin C, both active and inactive. For this reason, these forms may not be as potent or stable as L-ascorbic acid and may not absorb as quickly or be as effective. This is because inactive forms of vitamin C have to convert to L-ascorbic acid before they can be beneficial to the skin.

What Percentage of L-Ascorbic Acid Is Best?

For best results, it's recommended to use a vitamin C serum with at least 8% L-ascorbic acid. However, a higher percentage of L-ascorbic acid doesn't always mean the product is better—sometimes it can lead to skin sensitivity like stinging, redness and even breakouts.

"It is typically effective at concentrations between 10% to 20%. Higher than that would lead to irritation," says Khorana. "L-ascorbic acid is often formulated with complementary ingredients like ferulic acid and vitamin E to enhance stability and boost its antioxidant power," she adds.

Ideally a vitamin C serum with 10-20% L-ascorbic acid will deliver benefits to the skin, but if you have dry, dehydrated or sensitive skin or are new to using vitamin C, you should definitely opt for gentler serums with less than 15% L-ascorbic acid.

The Best L-Ascorbic Acid Serums

• Best affordable L-ascorbic acid serum: Naturium Vitamin C Complex Serum, £22
• Best high-end L-ascorbic acid serum: Skinceuticals C E Ferulic, £160
• Best L-ascorbic acid serum for mature skin: Allies of Skin 20% Vitamin C Brighten + Firm Serum, £93
• Best L-ascorbic acid serum for acne scarring: iS Clinical Pro-Heal Serum Advance+, £95
• Best L-ascorbic acid serum for dull skin: Drunk Elephant C-Firma Fresh Day Serum, £70
• Best L-ascorbic acid serum for dry skin: Jan Marini C-Esta Serum, £115
• Best L-ascorbic acid serum for oily skin: La Roche-Posay Vitamin C12 Serum, £45
• Best L-ascorbic acid serum for sensitive skin: Paula's Choice C15 Super Booster, £52
• Best all-rounder L-ascorbic acid serum: NIOD Ethylated L-Ascorbic Acid 30% Network, £37

1. Naturium Vitamin C Complex Serum

2. Skinceuticals C E Ferulic

3. Allies of Skin 20% Vitamin C Brighten + Firm Serum

4. iS Clinical Pro-Heal Serum Advance+

5. Drunk Elephant C-Firma Fresh Day Serum

6. Jan Marini C-Esta Serum

7. La Roche-Posay Vitamin C12 Serum

8. Paula's Choice C15 Super Booster

9. NIOD Ethylated L-Ascorbic Acid 30% Network