As far as subcultures go, bodybuilding is a famously tan one. Initially, Ken “Skip” Hill’s regimen was no different from that of other guys in the circuit. Supplementing tanning bed sessions with regular sun exposure secured him a solid char that helped contour his hard-earned muscles. “Brown fat looks better than white fat,” Hill explained. It was 1985, and the goal was to look like David Lee Roth, the lithe and atomically sun-kissed frontman of the hair metal band Van Halen. “I could sound like a Neanderthal,” Hill says from his current home in Southern Florida, where he works as a professional trainer, “but the muscle thing in my teenage years was: girls. The tan, for some reason, was every bit as important.”

In 2006, Hill was introduced, through other bodybuilders, to a peptide that stimulates melanin production, allowing users to achieve a much deeper tan with less sun exposure. “A lot of us are our own guinea pigs,” he said. Along with visits to the tanning bed, the peptide, called Melanotan-II, gave him the roasted-marrow tan he still has today.

Twenty years later, injectable self-experimentation isn’t just for bodybuilders, and being tan at all costs is trendy. Recently I noticed that—in spite of decades of mounting SPF awareness and sunless tanning innovation—tan lines were springing up as a cheeky summertime accessory. Soon after, my Instagram feed was overtaken: a guide for men on how to achieve the perfect tan (“Control your tan line narrative,” it said), a declaration that it is “Blue Crush summer,” featuring a sun-drenched still of bikini-clad surfer babes from the 2002 film; and several before-and-after photos of non-bodybuilders touting the perks of a good tan, powered by Melanotan II, or MT2.

Users claim the peptide gives them a leaner appearance, obscures blemishes, evens out their skin tone, and makes their teeth look whiter. Some report too-good-to-be-true purported side effects: appetite reduction, metabolism boost, heightened sex drive. Others opt for MT1, a related peptide that also promotes skin-darkening but doesn’t make you hornier or less hungry. Both chemicals seem to promise an eternal summer by making you perpetually look like you just got back from a long and expensive vacation—which perhaps explains how seamlessly it’s seeped into the wellness programs of non-bodybuilders.

“If you had asked me a couple years ago about injecting a Chinese chemical into my body, I would’ve been like, ‘What the fuck? That sounds insane,’” said Cale Airey, who works in marketing and uses MT1. Like most biohacking peptides, MT1 and MT2 are not approved by the FDA, and are illegal to sell for human consumption. But a 10 milligram vial (or nasal spray) can be purchased for roughly $40 from online retailers that label the chemicals, which largely come from factories in China, as for “research use only.” “Once I started looking into it, I became a lot more comfortable with the idea,” Airey said. “I just don’t really see it as a risk or a very big deal. And so, I just did it.”

This summer, the Skin Cancer Foundation put out a statement advising against MT2. “Case reports have documented the development of melanoma following Melanotan II use, as well as rapid changes in moles and pigmented lesions that can make the detection of skin cancer more difficult,” reads the warning, with reports also linking MT2 to systemic toxicity and muscle damage. “Ironically, you’re not in the sun now, but there are other problems. And the problems are the stimulation of the melanocytes, which are in everyone’s skin,” says Deborah S. Sarnoff, MD, president of the Skin Cancer Foundation and a clinical professor at NYU’s Department of Dermatology, contrary to the logic of looksmaxxers like Clavicular, who touted the peptide’s relative safety given the reduction in sun exposure. (“There’s absolutely no reason not to,” he suggested in a video posted in October.)