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 actually dates back to Ancient Egypt when Cleopatra would bathe in milk for softer, smoother skin, unknowingly reaping the benefits of an exfoliating acid found in sour milk—lactic acid.
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. 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. Later, in '96, when different concentrations were tested (5% and 12%), researchers observed that a higher concentration actually penetrated both the dermis and epidermis (5% could only reach the epidermis) for firmer, thicker skin, resulting in fewer fine lines and wrinkles. It's also been proven to reduce acne lesions, so yeah, you could say it's a bit of a hero ingredient.
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.
Side Effects of Lactic Acid
As with any acid, it's important to use it wisely. 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.
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 using lactic acid, and since sloughing off layers of your skin leaves you more prone to sun damage, make sure to apply an SPF of at least 30 daily (as you should be, anyway!).
The Best Products With Lactic Acid
Byrdie's editorial director says this scrub with exfoliating alpha-hydroxy acids (like lactic) makes her "forget" she has keratosis pilaris (a condition where your skin has little tiny bumps, usually on the backs of the arms and the thighs) for at least two days.
A Byrdie-editor favorite, this serum (which feels like a watered-down moisturizer) helps deliver an unreal glow. It still is an acid, though, so definitely follow up with a real moisturizer.
With a cooling gel texture, this ultra-hydrating serum sloughs away inactive skin cells while utilizing microalgae to replenish moisture.
The Ordinary is known for its high-powered serums with simplified formulas at an affordable price. This 10% lactic acid serum uniquely features purified Tasmanian pepperberry, which reduces signs of inflammation and sensitivity, a common side effect of exfoliation.
This combination of vitamins, enzymes, and amino acids preps the skin for forthcoming steps by removing dead skin and minimizing pores.
A personal favorite of this editor, this tonic is safe enough to use daily on clean skin. The result is that sort of cherubic glow you can only achieve after a facial.
Lactic acid doesn't have to be reserved for the skin on your face and body. It's also excellent for the scalp to exfoliate flakiness and strengthen the hair follicle. We love this particular
Touted as "the Hollywood 2-minute facial," this pore-unclogging treatment uses lactic acid and fruit enzymes to vacuum out congestion, leaving behind a smooth, bright complexion you'd think were only achievable by the hands of an esthetician.
For more info, check out our guide on how to incorporate acids into your routine.