The word “acid” conjures up images of bubbling test tubes and thoughts of scary chemical burns. But when used at the right concentrations, acids are actually some of the most beneficial ingredients available in skin care.

They’re the miracle tools used to fight acne, wrinkles, age spots, scarring, and uneven skin tone. But with so many acids on the market, it can seem overwhelming to remember which to use — and for what — and which products to buy. Before all that, you have to know where to start.

salicylic acid

Salicylic acid has been around for a long time. It’s well-known for its ability to exfoliate the skin and keep pores clear, which helps . You’ll find it in serums and cleansers at concentrations between 0.5 and 2 percent, as well as in spot treatments for breakouts.

Salicylic acid is also used in higher concentrations as a peeling agent for treating acne, acne scars, melasma, sun damage, and age spots in dermatology clinics. It’s so effective that it’s used in , although it’s still safe to use in pigmentation-prone dark skin. Since it’s related to aspirin (acetylsalicylic acid), it also has anti-inflammatory properties.

Popular salicylic acid products:

glycolic acid

Glycolic acid is the most popular alpha-hydroxy acid (AHA) used in skin care. It comes from sugar cane, and is the smallest AHA, so it’s the most effective at getting into the skin. Glycolic acid is a fantastic anti-aging agent that seems to do it all.

It’s very effective at , , fading dark spots, increasing skin thickness, and evening out skin tone and texture. So it’s no surprise that you’ll find it in many cult skin care products. It’s commonly found at concentrations below 10 percent.

Much like salicylic acid, glycolic acid is also used in , sometimes in tandem with microdermabrasion or microneedling. However, use of glycolic acid even when it’s not on the skin, so you need to use sunscreen as well to prevent extra sun damage.

Popular glycolic acid products:

Mandelic acid is another alpha-hydroxy acid, one that’s derived from bitter almonds. Like glycolic acid, it’s an exfoliating agent that’s useful for .

However, due to its larger molecular structure, it doesn’t penetrate skin as deeply as glycolic acid, so it’s less irritating to the skin. For this reason, it’s commonly recommended in peels instead of glycolic acid, which is more prone to rebound pigmentation. Rebound pigmentation occurs when a resistance is built up to a particular substance due to excessive use. This causes the substance to not only be ineffective, but often causes it to have the opposite of the intended effect.

Popular mandelic acid products:

Azelaic acid has been one of the mainstay treatments for fighting moderate acne for the last three decades, and is found in many prescription-only creams. It . It’s generally found at 15 to 20 percent concentrations in creams that are designed to be applied all over the face, morning and night. Azelaic acid generally has very few side effects, but in some people with very sensitive skin it can cause stinging, peeling, and redness.

As well as treating acne, azelaic acid is also a that’s useful for fading post-acne marks, or post-inflammatory hyperpigmentation. It’s frequently combined with retinoids as a milder alternative to hydroquinone.

Popular azelaic acid products:

Kojic acid is produced by bacteria used in the fermentation of rice for the production of sake. It’s a popular ingredient in Asian skin care products thanks to its . (Whitening is a term many Asian skin care brands use to refer to decreasing hyperpigmentation and uneven skin tone.)

It’s found in cleansers and serums at 1 to 4 percent concentrations. Unfortunately, it’s very irritating to skin — but it’s also very effective.

Popular kojic acid products:

Ascorbic is the most common water-soluble form of vitamin C, and is used in skin care for its anti-aging effects. It’s also been used as a in treating melasma. Ascorbic acid is very , so it’s commonly available in more stable forms under the name magnesium ascorbyl phosphate and .

Here are some other skin care acids that may be on the market. These acids may not be as popular, so they may be harder to find in common skin care lines and products, but there’s still evidence that they work:

lactic, citric, malic, and tartaric acidsAHAs that act as exfoliants, they also work to lighten uneven pigmentation and smooth out skin texture. Lactic acid is the best researched AHA after glycolic acid, and is notable for being gentler, more hydrating, and at treating sun damaged skin.
ferulic acidantioxidant ingredient that’s most commonly used in conjunction with vitamins C and E in serums. This powerful antioxidant trio is well known for its ability to from the damaging free radicals generated by UV radiation.
lipoic acidantioxidant ingredient with . Its effects are quite modest so its popularity is waning.
trichloroacetic acid (TCA)used in peels, and is especially useful for flattening out scars in the . It’s very potent and should be used by professionals only.
alguronic acidbyproduct of biodiesel production. It’s reported to have anti-aging effects, but these are yet to be supported by peer-reviewed research.

Linoleic acid and oleic acid, the assistants to transporting benefits

When talking about linoleic acid and oleic acid in skin care, it’s mostly in the realm of oils, where they aren’t true acids per se. In oils, these fatty acids have reacted to lose their acid groups, to form triglycerides. Generally, oils that contain more linoleic acid have drier textures that suit oily skin, while oils that contain more oleic acid feel richer and work better for dry skin.

Linoleic acid on its own has , but since it’s already found in oils, you’ll need to use a product that’s free of linoleic acid to achieve the same effect. Oleic acid on its own is a barrier disruptor that’s useful for helping drugs penetrate the skin.

Choosing which acid to use is the hard part. The easiest way to go about it, is by knowing what problem you want to treat.

Best for…Acid
acne-prone skinazaleic acid, salicylic acid, glycolic acid, lactic acid, mandelic acid
mature skinglycolic acid, lactic acid, ascorbic acid, ferulic acid
fading pigmentationkojic acid, azelaic acid, glycolic acid, lactic acid, linoleic acid, ascorbic acid, ferulic acid

Pro-tip: The higher the concentration, the more likely the acid will irritate the skin. Always patch test and start with a lower concentration before moving up.

Many acids offer multiple benefits and since they can come in many different formulations it’s possible to use more than one. Brands will often advertise the active acids in cleansers, serums, toners, and more, but check the ingredient list to make sure the acid is the active ingredient — listed near the top, and not a forgotten side character at the very end of the list.

What to know about mixing acids in your skin care routine

After your new shipment of beauty goods come in the mail, remember not to put them all on at the same time! Some acids may interact with others.

Don’t mix face acids

  • Don’t use salicylic acid with any other acid at the same time. Extreme skin irritation may occur when mixed.
  • Avoid salicylic acid with products that contain niacinamide.
  • Don’t use glycolic acid or lactic acid in combination with ascorbic acid (vitamin C). This will cause the ascorbic acid’s benefit to disappear even before it begins to work.
  • Avoid using AHAs with retinol.

To get around this, organize your acids between daytime and nighttime use. For example, use salicylic acid in the morning and another acid in the evening. You’ll still get the benefits of both if you use them at separate applications.

We pick these items based on the quality of the products, and list the pros and cons of each to help you determine which will work best for you. We partner with some of the companies that sell these products, which means Healthline may receive a portion of the revenues when you buy something using the links above.

Michelle explains the science behind beauty products at . She has a PhD in synthetic medicinal chemistry. You can follow her for science-based beauty tips on and .