FORTUNE — Lean is the name of a liquid dose of codeine and promethazine, cut with Sprite or Mountain Dew, topped with a Jolly Rancher, and typically served in a Styrofoam cup. Lean often comes from a purple prescription cough syrup made by Actavis — the color lends Lean its other popular nickname, purple drank, or purple jelly. It’s also known as sizzurp, oil, mud, and Texas tea, for it was popularized in Houston’s hip-hop scene. Lean is illegal, codeine is an opiate, promethazine a strong sedative, the cough syrup is a controlled substance, and the concoction is highly addictive and has killed severaloftherappers who made it popular.
And yet, on Instagram, it is relatively easy to locate plenty of individuals selling lean, just search #leanteam, #dirtysprite, #pourup. Fletcher Babb did and discovered “no less than 200 different accounts … of the ones that are real, many are not even hidden from public view.” His account of his communication with one Instagram drug kingpin who subsequently threatened his life using emoji is on the Vice blog Noisey, and it is astonishing. He also finds another selling with boxes of the prometh-codeine liquid, some bearing the logo of McKesson (MCK), ranked fourteen on the Fortune 500.
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Whether it likes it or not, Instagram has become a marketplace of goods, some weird and wonderful, some controversial, and — in the case of lean, or marijuana, or methadone — some straight-up illegal. Instagram’s terms, updated earlier this year, put the responsibility of all photo shares squarely on the user. (“You are responsible for any activity that occurs through your account …”) and by agreeing to them you agree not to break the law. But at a certain point, Instagram may be compelled to step in. It is, however, a double-edged sword. One of the largest gun busts in New York City history came about because of a post on Instagram.
And what, exactly, does the Facebook-owned (FB) company ban? An image of illegal drugs isn’t illegal — and may in fact be part of some user’s anti-drug statement. It’s all about context. Nuance is notoriously difficult to regulate, even in relatively simple interface like Instagram — just last week a controversy erupted over some pubic hair. For now, it is just fascinating to watch a new market emerge, one that isn’t off in some corner of the Internet, like the recently busted Silk Road, but right here in front of us, on our phones, in our lives.