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.
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 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.
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.
Now for 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. Using too much of an acid too often can lead to inflammation, which accelerates the aging process.
Also, consider discontinuing strong exfoliating products such as retinoid or scrubs when you're using a 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!). Ready to start using lactic acid? Shop our favorite products below.
Byrdie features editor Amanda Montell 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.
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.
This mix of AHAs and BHAs combines the gentle exfoliating power of the former with the deeply penetrating pore-unclogging prowess of the latter. It also helps promote skin elasticity for a more youthful-looking complexion.
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.
Harnessing the power of probiotics, this overnight cream feeds the skin good bacteria for a more balanced, hydrated complexion. It's also packed with lactic acid, of course, to polish the surface of the skin, along with vitamin C to boost collagen and lighten dark spots.
Touted as "the Hollywood 2-minute facial," this pore-uncologging 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.