So you finally decided to buy Biologique Recherche Lotion P50. The tipping point was, perhaps, yet another friend's rave review, or a Facebook ad from Rescue Spa, or maybe reading about it on this very website. Regardless, you've drank the lactic acid Koolaid, and we salute you for your astute and well-guided consumerism. But now you're confronted with a brand-new conundrum: There is a small universe of P50 types. Which one is the right one for you?

We've talked about P50 for ages, but we've never really dove into what makes it so good, until now. For the uninitiated, Lotion P50 isn't a lotion at all—it's a French waterweight exfoliating toner that also hydrates and balances your skin's pH, all in one fell swoop. It also smells like formaldehyde and apple cider vinegar, a scent Emily Weiss lovingly described as "trash face." But it's got a cult following. And if you're in that cult, you know that one of the only places you can get it online is the website for Philadelphia's Rescue Spa. "It boosts your skin's very slow, very inefficient natural exfoliation process by immediately removing bad stuff from your skin," says Danuta Mieloch, owner of Rescue Spa and person more or less to thank for its popularity. (Well, her and Aida Bicaj.) She recommends different versions of P50 for different skin types, and was happy to break them down for us:

For masochists: Lotion P50/P50 1970

P50 1970 is the toughest, roughest, OG version. Stinging and redness is par for the course—but actually, it's good for any skin type. It’s got all the good stuff you know and love about P50, including a hearty blend of AHAs and BHAs to give you that glow. But what makes P50 particularly special is the mix of sorrell, myrrh extract, myrtle, and onion. (That's where the smell comes from.) The regular P50 (scratch the 1970) excludes phenol, which is also found in mouthwash. It’s been banned in Europe, but it's all part of the product that Danuta describes as the "purist's choice."

For dry skin: Lotion P50V/P50V 1970

P50V was created for those with skin not too sensitive, not too strong, and perhaps on the normal to dry side. It still does the job, but with the addition of plankton and brewer’s yeast, it's slightly more conditioning than regular P50.

For super sensitive skin: Lotion P50W/P50W 1970

Or: For when you want effective exfoliation, but just looking at the word "glycolic" flares your skin up. P50W is the best bet—the formula balances lactic and salicylic acid with glycerin and articum lappa root to make it effacaious and gentle. How many products can do both?

For hyperpigmentation: Lotion P50 PIGM 400

The newest member of the P50 line, which contains no phenol at all, was made specifically for people dealing with hyperpigmentation. This version helps control the pigmentation process, stimulating exfoliation and helps brighten dull skin with wasabi extract, palmaria palmata extract, and whatever "fruits titrated in flavonoids" are. Dr. Phillippe Allouche (he helped develop the line) expressed the risk of our skin becoming dull due to external stressors—sun, stress, city air, etc. Absolutely worth checking out if skin tone is your #1 concern, but not as strong as regular P50. But then again, few things are.

Photographed by Tom Newton.