If the two things you can't live without are a dark, even tan and a fast-acting, long-lasting erection, then add Melanotan II to your holiday shopping list. This synthetic peptide hormone, which was developed by a research team at the University of Arizona during the late 1990s, darkens skin pigment and may stimulate erectile activity. And despite continued concern and controversy within the medical community regarding its use, it remains available for sale over the internet in a powdered form that can then be reconstituted for subcutaneous injections.

Melanotan II first captured the public's imagination when the mainstream media briefly touted it as a Viagra-like panacea for middle-aged men. Norman Levine, a dermatologist who led the team that developed the drug, had first conducted experiments in which he darkened the pigments of frogs by injecting them with a hormone called Alpha MSH. After Melanotan II was modified for human use and put through clinical trials, Levine reported that "one very astute observer who took this drug told us that he was developing spontaneous erections."

During the 2000s, the Melanotan II peptide and the metabolite derived from it, the erectile dysfunction-focused Bremelanotide (also known as PT-141), were patented and then licensed to biotechnology companies hoping to develop them into profitable prescription drugs. However, these companies also offer the peptides for direct sale to researchers. These transactions occupy a legal gray area, since the peptides are banned for human use outside clinical trials. While they can be purchased from various websites specializing in research chemicals, the purchaser usually has to affirm prior to final sale that the peptide "will not be used for human consumption" and is being acquired for "research purposes only."

Although obtaining Melanotan II and Bremelanotide is relatively easy to do, both substances come in powder form and then must be reconstituted using sterile water prior to subcutaneous injection—a method of administration that can cause lead to skin bruising, cross-contamination, or infection, if the person performing the injection is inexperienced and the syringe isn't clean.

Despite those drawbacks, both Melanotan and Bremelanotide now enjoy considerable popularity within certain subcultures, such as the "gay muscle" scene.

"You've got this stuff, particularly PT-141—which causes you to want to have sex with everybody and everything—really making the rounds," bodybuilder Zander Wiebe told Motherboard. "People are incorporating it into their sex parties, along with other performance-enhancing drugs."

Research substantiates Wiebe's anecdotal claims. Melanotan.org, a forum dedicated to the peptide that shut down in 2011, had thousands of regular posters, many of whom have since migrated to other discussion boards. In 2009, a BBC report tracking just six needle exchanges found that hundreds of individuals had visited these exchanges in order to receive syringes for Melanotan II use.