If someone told you that applying spoiled milk to your skin could get rid of blemishes and diminish fine lines, would you believe them? On the surface, it sounds like a weird Pinterest hack, but the science serves as proof. It dates back to Ancient Egypt when Cleopatra bathed in milk for softer, smoother skin, unknowingly reaping the benefits of an exfoliating acid found in sour milk—lactic acid.
Now, we're not suggesting you splash sour milk on your face nightly, because that would be ridiculous and incredibly unpleasant. Instead, beauty brands have caught on and begun to isolate lactic acid, integrating it into serums and cleansers for the consumers' benefit. Dhaval Bhanusali of Hudson Dermatology & Laser Surgery explains that lactic acid is now known as "One of the more common alpha hydroxy acids," adding that it's "used to improve skin tone and texture as well as lightly exfoliate the skin." We consulted Bhanusali and other experts for a full perspective.
Meet the Expert
Type of ingredient: Acid/exfoliator
Main benefits: Firmer, thicker skin, resulting in fewer fine lines, wrinkles, and acne lesions.
Who should use it: In general, anyone with acne-prone skin or who frequently breaks out.
How often can you use it?: It's only recommended for use once a day at maximum, but should probably be used once every few days.
Works well with: Hydrating ingredients like hyaluronic acid.
Don’t use with: Other acids and exfoliants like AHAs or benzoyl peroxide. Retinol should be avoided as well.
What Is Lactic Acid?
Swedish chemist Carl Wilhelm Scheele was the first to isolate the compound from sour milk, and in the late 1800s, German pharmacist Boehringer Ingelheim uncovered how to mass-produce lactic acid when he realized it was a byproduct of fermented sugar and starch in sour milk via bacteria. "Lactic acid is a light peeling agent, depending on strength," says MacGregor. She also notes that it "can smooth out the skin, making it glow." You can find lactic acid in many of the same places other Alpha Hydroxy Acids (AHAs) are found, including products that advertise that they contain AHAs but don't specify which they contain.
Benefits of Lactic Acid
- Kills bacteria: Based on our Google deep dive, research around lactic acid's efficacies as they relate to the skin are limited in the early to mid-1900s, but in 1985, a study found that lactic acid helped to kill skin infections in newborn infants. It tracks that it would do the same in adults, including the irritating, acne-causing kind.
- Diminishes wrinkles: Later, in '96, when different concentrations were tested (5 percent and 12 percent), researchers observed that a higher concentration penetrated both the dermis and epidermis (5 percent could only reach the epidermis) for firmer, thicker skin, resulting in fewer fine lines and wrinkles.
- Reduces acne: It's also been proven to reduce acne lesions. So yeah, you could say it's a bit of a hero ingredient.
- Increases cell turnover: According to Schep, it "works by increasing the rate of skin turnover, causing new and younger-looking skin to grow."
- Helps skin hold moisture: Similarly, Schep says that "The new skin may also have better moisture holding capability."
Lactic Acid vs. Other Acids
If you're wondering how it differs from other acids, like, say, glycolic acid, the lactic acid molecule is actually larger, so it can't penetrate as deeply—instead, you're getting more surface treatment (polishing, firming, exfoliating goodness). This is good news for those with sensitive skin, though, who'll likely be able to tolerate its effects better. This isn't just good news for people with sensitive skin, though, because it means lactic acid is also less likely than glycolic or salicylic acid to cause irritation and disrupt the pH of your skin barrier. So pretty much anyone can use it. If you're someone with acneic skin who needs a lot of exfoliation, you can alternate your use of lactic acid with the use of something deeper like salicylic, which will clear off dead skin and deep clean your pores.
Side Effects of Lactic Acid
As with any acid, it's important to use it wisely. "Do not use on irritated or red skin!" MacGregor warns. Using too much of any one acid too often can lead to inflammation, which accelerates the aging process. Using too much at once can also lead to inflammation or—worse—rashes and chemical burns. It is worth noting, though, that lactic acid actually does irritate skin less than many other acids, and therefore, can be used more often than something like (the very intense) salicylic acid.
How to Use It
Celebrity esthetician Renée Rouleau recommends incorporating a lactic acid product (like a serum or toner) on a three-on/three-off schedule, i.e., you should apply the acid for three nights in a row, then take a break for three nights to treat your skin with hydrating ingredients that nourish the new cells you've revealed.
Also, consider discontinuing strong exfoliating products such as retinoid or scrubs when you're regularly using lactic—or any—acid. "As it makes new skin grow, which may not have any pigment in it, it may cause increased susceptibility to sunburn. Therefore, it is often formulated with sunscreen," says Schep. Since sloughing off layers of your skin also leaves you more prone to sun damage, make sure to apply an SPF of at least 30 daily (as you should be, anyway!).
Who should use lactic acid?
Anyone with acne prone skin. Just remember to do a patch test first, to detect any side effects or further irritability to the skin.
What is lactic acid best known for?
Lactic acid is best known for its exfoliating properties, however, it boasts other benefits as well.
Naval T., Aligui G., Rivera E. A study of bacterial skin colonization among newborn infants using lactic acid and hexachlorophene. 1985.
Smith WP. Epidermal and dermal effects of topical lactic acid. J Am Acad Dermatol. 1996;35(3 Pt 1):388-391. doi:10.1016/s0190-9622(96)90602-7
Rathi SK. Acne vulgaris treatment : the current scenario. Indian J Dermatol. 2011;56(1):7-13. doi:10.4103/0019-5154.77543
Soleymani T, Lanoue J, Rahman Z. A practical approach to chemical peels: a review of fundamentals and step-by-step algorithmic protocol for treatment. J Clin Aesthet Dermatol. 2018;11(8):21-28.