Hollywood stars love their eccentric facials, and it often seems like they’ll put anything on their faces. The latest buzzy ingredient? Foreskins. Yes, you read that right.

Last week, Sandra Bullock appeared on “The Ellen DeGeneres Show” and revealed that she and Cate Blanchett are fans of what they’ve dubbed the “penis facial.” We’ll admit, it sounds more X-rated than luxurious.

The treatment involves the use of something called epidermal growth factors, or EGF for short, which are derived from stem cells taken from the discarded foreskins of newborn babies in Korea. EGF is said to help rejuvenate the skin, improve overall skin texture and correct discoloration. It’s also known for its ability to aid in wound healing.

Um, what exactly is the “penis facial”?

The specific facial both Blanchett and Bullock got is called the Hollywood EGF facial treatment from The Georgia Louise Atelier in New York, which started offering the facial last year. The treatment will cost you at least $650.

A representative for Louise’s atelier told HuffPost in an email that the facial consists of a TCA chemical peel, followed by micro-needling. Micro-needling, as Dr. Samer Jaber of Washington Square Dermatology told HuffPost, involves a process of creating tiny holes in the skin with a special tool (essentially wounding the skin), to promote healing and the creation of new collagen. The treatment also allows for better, deeper product absorption, he said.

Dr. Rachel Nazarian, of Schweiger Dermatology Group expanded on the treatment, telling HuffPost the absorption of products is “exponentially increased” after micro-needling, which helps “trigger anti-aging much deeper, which is where you want it.”

Following the micro-needling, Louise uses a special “electrifying” mask to calm the skin. Then comes the “penis” aspect, what Louise calls her “secret box” of EGF serum. The EGF is “derived from the progenitor cells of the human fibroblast taken from Korean newborn baby foreskin ― which helps to generate collagen and elastin,” the atlier’s rep added.

Louise said in an email that she’s mindful of informing her clients about where the EGF comes from. (For what it’s worth, Louise’s serum is FDA approved.)

“I always explain that EGF is derived from newborn baby foreskin, BUT cells were taken and from that, new cells are cloned from a laboratory,” she said. “The EGF serum only contains cloned cells.”

Nazarian said that once an initial stem cell has been taken from the source, in this case a discarded foreskin, “you don’t need to keep taking it. You just need the one initial [stem cell] and you can keep growing it forever and ever and ever.”

The use of EGF in skincare treatments and products isn’t particularly new, Nazarian said. In fact, she said, professionals have been using it for years with mirco-needling, among other applications. Beauty brands, such as DHC, Mizon, and Peter Thomas Roth also sell products that use it.

What’s so special about EGF?

EGF is a polypeptide (a chain of amino acids) found in the skin’s fibroblast that promotes cell growth, development and healing, according to skincare brand Bioeffect, which uses a form of EGF in some of its products. In 1986, scientists Stanley Cohen and Rita Levi-Montalcini were awarded the Nobel Prize for their discoveries of growth factors.

A study that looked at the role of EGF in cosmetic products and treatments noted that while topically applied EGF and other growth factors have been shown to rejuvenate the skin, their molecules are actually quite big, which can make it hard for them to penetrate the skin. That’s why micro-needling is involved, opening a path for better absorption.

Another small study, which used EGF derived from barley instead of foreskins, found that use of an EGF serum twice daily for three months improved the appearance of brown spots, pore size, wrinkles and even overall skin texture.

The penis facial is a little controversial. Here’s why.

Questions regarding the ethics of cultivating EGF or penis-derived cells have been raised over the years. As Quartzy pointed out, Oprah once endorsed a Skinmedica cream containing stem cells derived from baby foreskin, sparking controversy with at least one anti-circumcision activist group.

The newly piqued interest spurred by Bullock’s recent “Ellen” interview has sparked criticism from another anti-circumcision group, Intact America.

“That Bullock and DeGeneres could joke about using foreskins, taken without consent from Korean baby boys, for a beauty treatment indicates a total lack of understanding of the ethics involved,” Intact’s executive director, Georganne Chapin, said in an emailed statement.

Baby foreskins aren’t the only source of EGF.

While the EGF in many beauty products appears to come from Korean babies, it should be noted that the growth factor can also be found in kidneys, urine and saliva. Alternatively, Bioeffect uses an EGF derived from barley.

Nazarian said that snail excretions ― another buzzy beauty ingredient, especially in Korean skincare products ― also contain a growth factor similar to that in baby foreskin.

However, she said a growing group of dermatologists (herself included) believe that EGF derived from human skin can provide the best results for their clients.

“You’re putting it on human skin, so you’d want something that is more identical to what the human body would do to itself to heal,” Nazarian said. “I used to use a lot of snail secretions for my growth factors and stem cells, but now I’m switching over to more human.”

The reason dermatology favors EGF derived from foreskins, as opposed to saliva or urine, isn’t totally clear. Nazarian explained that while growth factors derived from saliva or urine could theoretically be used in skincare, “they don’t have exactly the same functions as those from skin.” They do, however, overlap in their abilities for wound healing.

“I don’t have a clear explanation as to why they’re not used, and why foreskin is preferred in dermatology, but it’s likely because it’s more directly related to the type of cell we’re targeting (skin!) and it’s so easy to harvest (as opposed to salivary gland cells or other options),” Nazarian said.