"In need of Lantus will trade Novolog," read a post in one of the forums. "I have Lantus pen and I'll trade you as I need Novolog! PM [private message] me," said one of the replies. Another reply suggested the shipper use "a frozen juice box or ice pack" to keep the biopharmaceutical refrigerated.
"ISO [in search of] a Novopen Echo. I have Humalog and Apidra pens and Apidra vials and Ominpods to trade," read another post. A match was made when another forum member replied, "I have one with some unexpired Novolog refills. I am in need of Apidra pens!"
Membership in the mostly private groups may require moderator approval. NBC News reporters requested to join and were admitted to some of the groups. Their member numbers can range from a few dozen to a couple thousand.
The posts take various forms. Sometimes a patient asks for help when they've run out of supplies, insurance coverage and/or money.
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Another person who has extra to spare might arrange to send theirs to them, only asking they pay shipping and handling in return.
Others will offer to exchange their supplies for something else they need. For instance, someone whose insurance only affordably covers Humalog but they feel Novolog works best for their body will post a trade. Then someone in the opposite position, whose insurance covers Novolog but they would rather have Humalog might respond -- and they'll swap, mailing each other their drugs.
Patients will sometimes even arrange to meet in person.
It's not just insulin either. NBC News searched Facebook and found postings to trade EpiPens, asthma inhalers, and other prescription medications.
Not every exchange goes seamlessly. Judging by moderator comments and FAQ's, it's not uncommon for deals to fall through or shipments to be delayed.
But participants say the dangers, which doctors and regulators say include tampering, infection, privacy issues, and scams, are worth the alternative — a visit to the emergency room, or worse.
"If it weren't for the online diabetes community I would be dead," said Amy Leyendecker, a 43-year-old medical transcription student from Kentucky living with Type-1 diabetes who requires daily doses of insulin to stay alive.
When her husband switched jobs and insurance plans, she panicked when the new plan wouldn't cover her brand of insulin, Humalog, made by Eli Lilly. Her costs shot up from under $25 a month to over $900 according to receipts from her pharmacy — unaffordable for the blue-collar single-income family living paycheck to paycheck. In a panic, she reached out online and a woman she met through a diabetes support group on the internet mailed her some of her own insulin, Novo Nordisk's Novolog.
Her primary care provider, Brandon Wampler, a physician assistant with Heartland Primary Care, in Elizabethtown, Kentucky, told NBC News that he "cautioned her to be leery," to make sure she trusted who was sending it to her and to check the tamper seals on the vials.
"You don't know if something has been contaminated or poisoned," he warned.
But she was undaunted. "She wasn't asking for my permission or opinion, she had already figured it out," said Wampler. "These are certain risks that desperate people are trying to take."
In case an "axe murderer" showed up after she shared her address online, she took the precaution of telling several friends about the deal. When the shipment of insulin arrived, she checked to make sure the vials were sealed. She only took a small dose at first to try it out. But the shipper had lived up to her word.
Leyendecker was covered for the next six months, injecting someone else's insulin prescription into her body.
"All the supplies in my house have been donated to me," she said. "I feel like I'm living on borrowed time. My life is for profit."
Since then she's been able to go the doctor and get a new prescription under her new insurance plan. But with a new move and new job coming up and insurance coverage always in question, she could find herself back on Facebook.
Doctors say patients like Leyendecker take a big gamble.
"Patients can put themselves in grave danger by using insulin 'traded' online," said Dr. Joshua Miller, medical director of diabetes care at Stony Brook Medicine, running the risk of infection, or fluctuating blood sugar levels if the insulin was expired or stored incorrectly. He said several of his patients have gone to the hospital after using expired insulin.