Kelsey Weier of BIP says it works, so we took it to dermatologists for their thoughts.

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Bachelor in Paradise returned on Monday night after a long pandemic hiatus, and it was back to business as usual for the people on the show. Namely, the reality stars tried to couple up with each other while staying cool under the hot Mexican sun.

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Credit: Getty Images

While most people on the beach ended up sweating buckets, there was one in particular who was determined not to let her makeup melt: Kelsey Weier. When she first arrived at the Playa Escondida resort, Weier shared with bartender Wells Adams  that she tends to sweat a lot. So she decided to apply milk of magnesia to her face, a hack she says she learned from a makeup artist.

People immediately flooded Twitter with comments.

OK, but…what?! Here's what you need to know about this supposed beauty hack.

What's is milk of magnesia?

Milk of magnesia is a liquid medication that's used for a range of digestive issues, including constipation, heartburn, and upset stomach. The active ingredient in milk of magnesia is magnesium hydroxide, which is a saline laxative. Meaning, it makes you poop.

Milk of magnesia works by drawing fluid into the intestines, which helps move things along down there, dermatologist Gary Goldenberg, MD, of New York City's Goldenberg Dermatology, tells Health.

Can milk of magnesia stop sweating?

FWIW: There's nothing on the label of popular forms of milk of magnesia that say it's supposed to be slathered on your skin. Still, dermatologists say this ~might~ work to prevent sweating. "Applying it to the skin could draw water in the form of sweat from the skin," Dr. Goldenberg says. Whether that would actually stop you from sweating, though, is debatable, he says.

Magnesium-based salts are also used in many natural deodorants to help neutralize body odor, Joshua Zeichner, MD, director of cosmetic and clinical research in dermatology at Mount Sinai Hospital, tells Health. "Since it has antimicrobial properties, milk of magnesia can help lower levels of odor-causing bacteria on the skin."

"Sweat is usually odorless, but when it is broken down by bacteria that normally live on the skin, it can cause a foul smell," he continues.

As a result, Dr. Zeichner says, it could technically be used under your arms or other areas where you get sweaty.

Is it a good idea to use milk of magnesia on your skin?

Eh…dermatologists aren't exactly on board with this hack. "I do not recommend using it," Dr. Zeichner says. "Cosmetically, it is extremely unpleasant to apply on the skin. It would need to be applied as a mask, left to dry, then washed off." (And that would defeat the whole purpose of using it to block sweat.)

Milk of magnesia also has an alkaline pH, "which can disrupt the outer skin layer, leading to dryness and irritation," Dr. Zeichner says.

Milk of magnesia may also clog pores, leading to acne breakouts, or even cause an allergic reaction, Dr. Goldenberg asys.

Putting on makeup over milk of magnesia probably isn't as easy as it sounds, Dr. Goldenberg says. "It's a liquid substance, so it would be difficult to apply something over it," he says.

The overall takeaway? Don't try this trick at home.

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