The authors, led by Dr. Sarah Steele, a professor of the Global Health and Policy Unit at Queen Mary University of London, say breast milk won’t improve health or sports performance. There’s less protein in human breast milk than cow’s milk, and any adult who claims a health boost from breast milk is probably experiencing a textbook case of the placebo effect, say the authors.

It’s not so surprising that some people have come to believe that a few glasses of mother’s milk could be a magic elixir for an adult. Plenty of research shows that breast milk is more beneficial than formula for a baby’s physical and cognitive development. One study found that breastfed babies are even more likely to turn out better educated and earn more money as adults.

Most mothers selling their goods on the black market aren’t concerned with quality control, which means many don’t sufficiently sanitize before pumping. Lack of refrigeration during transport also provides ample opportunity for the milk to become contaminated with bacteria that may cause foodborne illness.

Additionally, black market breast milk may contain other pathogens that cause serious infectious diseases, including hepatitis, syphilis and HIV; many of these diseases do not have symptoms, so a woman may even be unaware that she’s sick when she sells her milk.