A few days after Cassie Conley gave birth to her third child, Peter, in 2014, he began to have worrying reactions to his formula.

"It started with a rash," she recalls. "It was everywhere." He soon began vomiting after feedings, and his weight plummeted.

Switching Peter to only breast milk wasn't an option; Conley's milk supply was too low to sustain him. She had successfully relied on a combination of breast milk and formula to feed her other two children when they were infants—but after trying several different types of formula for her youngest baby, she couldn't find one that Peter was able to tolerate. So Conley, her lactation consultant, and her pediatrician came up with a new plan: Conley would need to supplement her milk with another mother’s in order to feed her son.

Milk sharing between mothers is nothing new, but it has garnered more attention in recent years, thanks in part to peer-to-peer milk donation networks like Eats on Feets and Human Milk 4 Human Babies (HM4HB), informal communities that connect mothers who make a surplus of milk to mothers in need. The donors are encouraged to provide whatever they can, free of charge. Many members of these milk-sharing communities place a high value on breast-milk exclusivity, contending that any breast milk at all—even milk that doesn't come from the baby's mother—is better than processed formula.

By the time Emma Kwasnica, a self-described breastfeeding advocate with thousands of Facebook followers and the founder of HM4HB, created the network five years ago, she had already been connecting dozens of donors and recipients on her personal Facebook page for months. In 2010, she decided the time had come to organize these connections into something larger. Kwasnica rallied her Facebook followers to set up their own local milk-sharing pages online as the first chapters of HM4HB. As more followers volunteered to organize chapters, the network quickly grew to 60 chapters by the end of the year. Today, HM4HB has a global presence, with 130 chapters in more than 50 countries.

The American Academy of Pediatrics and the World Health Organization both recommend that mothers exclusively breastfeed their babies for the first six months, and past studies have shown that babies who breastfeed may be at reduced risk of health problems, including gastrointestinal problems, allergies, and diabetes. But while health professionals agree that breast milk is nutritionally superior to formula, research on the long-term impact of breastfeeding has yielded mixed results. A study published last year in the journal Social Science and Medicine, for example, found that many of breastfeeding's benefits may be due to other factors, like mothers' socioeconomic status. (And last month, The Atlantic argued the same thing about a more recent study about breastfeeding and IQs published in The Lancet).

And, when the breast milk comes from another mother, health organizations are more skeptical about its value to babies. In response to the growing popularity of milk-sharing, the Food and Drug Administration released a statement in late 2010 that listed the possible dangers of shared milk, including exposure to infections like HIV, chemical contaminants such as illegal drugs, and a number of prescription drugs that doctors recommend mothers avoid while breastfeeding. Improper storage or handling of breast milk can also introduce environmental contaminants and bacteria. A number of other health organizations released similar statements soon after the FDA, including the American Academy of Pediatrics and La Leche League International.

Clay Jones, a pediatrician at Newton-Wellesley Hospital in Newton, Massachusetts, regularly advises mothers who struggle to breastfeed, helping them to assess the pros and cons of formula, milk obtained from a milk bank, and donor milk—and always steers them away from the last option.

"Weighing risks and benefits of medical interventions is something that I do daily as a physician," he says. Citing the risk of exposure to viruses and drugs, he adds, "My personal stance is that there is no situation where the benefits of online or acquaintance donor milk outweigh the risks." To date, there have been very few recorded instances of babies in the United States being harmed from peer-shared breast milk. In 1977, human donor milk sickened seven of the 22 babies at an unspecified neonatal intensive-care unit. The milk, which was traced back to a single source, tested positive for salmonella.

Jones says that, thankfully, "I have never had a patient suffer a bad outcome related to donor milk from any source." Even so, he advises patients to avoid unnecessary risk and use banked milk that has been tested for contamination, pasteurized, and appropriately stored.

"The notion of 'informed choice' is at the heart of this set-up," Kwasnica explains, adding that, to the best of her knowledge, no babies have been harmed through milk-sharing in the HM4HB community. Kwasnica acknowledges that there is a level of risk involved, but she believes that families can make their own informed choices about whether the benefits of milk-sharing outweigh the risks. HM4HB doesn't mention the specific risks on its website, though it does encourage each recipient to screen donors, ask for copies of the donors' medical records, and consider pasteurizing the donated milk at home.

After doing her own research on donor milk, Conley decided to take what she describes as a "calculated risk." Although her pediatrician counseled her to obtain donor milk through a bank, Conley still needed a way to feed her son during the months it took her insurance company to authorize payment for the cost of banked milk. Through her lactation consultant, Conley was introduced to her first donor.

Conley's method of finding a donor is typical: Women rarely seek out milk from complete strangers, according to Beatriz Reyez-Foster, an anthropologist at the University of Central Florida who specializes in donor-milk research. Mothers who share milk through these less formal networks primarily use the honor system to vet potential donors, relying on word of mouth from acquaintances.

"Usually the donor and recipient move in the same social network," she says. "If they don't know each other, they at least have people they know in common." Many donors, she explains, feel that this acts as a safeguard against some of the risks inherent in milk-sharing.

Erika Gebhart, a doula and mother of five from Michigan, chose to use donor milk for her youngest child when her own milk supply tanked after gastric-bypass surgery in 2008. Gebhart didn't want to use formula, so she turned to social media, reaching out to other mothers she knew—ones with chubby babies and a surplus of breast milk—to help her. "I was really plugged into the birth community, and I knew a lot of people who were militant breast-feeders," she says. "I knew a lot of people with extra milk."

Over the next 12 months, Gebhart received free breast milk from around 15 different donors, reaching out through places like Craigslist and parenting support groups on Facebook. Gebhart relied on an honor system from her donors, who, at her request, disclosed their medications and dietary habits before donating. "I took milk from donors who used Zoloft, because I had used Zoloft and nursed my own baby," she says. "Hopefully they weren't shooting heroin, but I chose to believe that if they're taking time out of their day to pump extra milk, they have the children's best interests at heart."  

Research has shown that many women who choose to become donors do so for altruistic reasons. In a 2014 survey published in the journal Breastfeeding Medicine, 73 percent of breast-milk donors said that they donated excess milk because they wanted to help someone. (Other reasons included not wanting to waste an excess of breast milk, or protesting the high cost of banked milk out of solidarity.) "I read an article about a mother who could not nurse because of health problems and her son was allergic to formula," one woman said. "I couldn’t imagine having that happen and the feelings of helplessness. So I looked into donating."

"[HM4HB] was born out of the empathy I felt for other mothers who couldn't breastfeed," Kwasnica agrees. She says that although she's unable to express milk for private donation, she's nursed the babies of several friends when their mothers weren't able to do it themselves. "I don't respond well to a pump," she says. "But I'm happy to feed any baby in need at my breast."

But like the FDA and others have pointed out, despite good intentions of most milk donors, many may not be aware that their milk could be contaminated. Some diseases that can be transmitted through breast milk, such as HIV and cytomegalovirus, can have an incubation period of several months to several years. In a 2012 retrospective study in the journal Pediatrics, researchers found that of the who tried to donate to the Mother's Milk Bank in San Jose, California, from 2000 to 2005, 1.5 percent came up positive for syphilis, HIV, HTLV-1 or 2, or Hepatitis B or C during the bank’s milk-testing process. (Their donations were discarded before they could be given away.) With these results even in banked milk, the authors concluded, peer-donated milk—which is unpasteurized and often comes from untested sources—"may pose a significant health risk to exposed infants."

Gebhart had a similar issue when she accepted donor milk from an acquaintance several years ago. "We had babies the same age. They had the same birthday," she recalls, "and I friended her on Facebook after she gave me some donor milk." Weeks later, the donor posted an angry status on her Facebook page, claiming that she had just discovered her husband had been cheating.

Gebhart was horrified. "I wondered, is there a chance that her husband could have given her some disease? Could it have gone into the milk and into my baby?" When she contacted the donor, the woman assured her that her STD panel was clear. Because her child never showed any troubling symptoms, Gebhart took the woman at her word and didn't get the baby tested.

It's worth noting that the risk of contamination is increased significantly if the milk is sold online rather than freely donated or obtained from a milk bank. In a 2013 study, the American Academy of Pediatrics purchased more than 100 samples of breast milk off of Craigslist and compared it to 20 samples that had been rejected from a milk bank due to contamination. An analysis of the purchased milk showed a much higher incidence of staphylococcus and streptococcus contamination than the banked samples. And earlier this month, a separate study published in Pediatrics found that nearly 10 percent of purchased breast-milk samples contained cow DNA. Sarah Keim, a professor of pediatrics at the Ohio State University and a lead researcher in the recent Pediatrics study, wrote in the paper that some women who sell their breast milk may dilute it with formula or cows' milk for greater profit.

For this reason, neither Eats on Feets nor HM4HB supports or approves of the sale of breast milk within their communities. "If someone is attempting to make money by selling breast milk, how do we even know that what they are selling is actually 100 percent breast milk?" the Eats on Feets blog asked in response to the research, adding, "It is our firm belief that the selling and buying of breast milk carries undue medical and ethical risks."

Compared to peer donation, milk banks generally come with significantly less risk (even if, as the 2012 Pediatrics study shows, they aren't entirely risk-free). The Human Milk Banking Association of North America (HMBANA) details its donor guidelines and sterilization process on its website: Milk from several different donors is mixed together and then pasteurized in large batches to rid it of bacteria. Donors typically need blood tests and a note from their physician to ensure that they are in good health. These blood tests and doctor visits come at the expense of the donors, who are not compensated for their milk.

But despite the risks of peer-to-peer milk sharing, many mothers still find it a more appealing alternative than milk banks, which can be expensive. When Conley's doctor suggested she used banked milk, she discovered that it would cost her $3 per ounce. For comparison, one 23-ounce container of Similac formula typically costs roughly $21 on Amazon, or $0.91 per ounce. If the family's insurance plan hadn't agreed to cover it, a six-day supply of banked breast milk would have cost around $600.

Gebhart also researched milk banks before deciding that they were "cost prohibitive," she says, adding that her child likely wouldn't have qualified for banked milk, anyway. Milk banks typically require recipients to have a prescription, and doctors may not write prescriptions for donor milk unless the child is premature or otherwise medically fragile. For healthy babies, Jones says, there is a lack of evidence that donor milk is any more beneficial than formula, and for that reason, some doctors can be hesitant to prescribe it for the sole reason of accommodating a mother’s preference. "I believe that [banked donor milk] should primarily be reserved for use by hospitals," Jones says. Even then, demand often outstrips supply, and banks usually prioritize hospitalized babies.

"Even though we got milk from the bank," Conley says, that "doesn’t mean [the milk bank] can supply us with everything we need. Most of the time we still have to search for donor milk to cover what they can't." Before Conley's youngest son started solid foods, he drank up to 32 ounces of breast milk on a typical day. The milk bank, despite Conley's prescription, could only provide her with 24 ounces per day, at most, she says, because most of the supply was reserved for babies with severe medical problems. "They give you whatever they can spare," Conley says.

Until her son's first birthday, she fed him a combination of her own breast milk, peer-shared donor milk, and banked milk (and, in recent months, solid food). At Peter's 12-month checkup earlier this month, the pediatrician delivered some good news: His weight had stabilized, and he no longer had any trace of the rash or gastrointestinal issues associated with his formula allergy.

Today, Peter is a healthy one-year-old, and Conley no longer has to rely on donor milk—but, she says, she's happy with the choice she made: "I wanted [Peter] to have the best I could offer," she says, even if it carried a risk.