Parents who buy human breast milk over the Internet may receive a product mixed with cow’s milk that could cause an allergic reaction in infants, according to a study published Monday in Pediatrics. The results suggest that some mothers who sell their leftover breast milk online are mixing it with cow’s milk to stretch the supply and earn more money.

Moms who have leftover breast milk after feeding their newborn can sell it through sites such as Only the Breast to parents who can’t produce milk of their own and those who are taking medicine that might be harmful to their baby through breastfeeding. Sellers can fetch around $1 to $3 an ounce for the milk. Many online listings include information about the seller’s diet and lifestyle and tout the health of their own infants. Some sellers include photos of their babies as evidence that their milk is particularly nutritious.

“The truth of the matter is that you do not truly know what you are receiving when you buy milk from a stranger over the Internet -- no matter how chubby their baby is or how many claims they make about their organic or drug-free or ultra-healthy lifestyle,” Sarah Keim, a researcher from Nationwide Children’s Hospital in Columbus, Ohio, writes in a blog post.

Keim and her team analyzed 102 samples of breast milk for the study and found that 10 of those samples also contained cow’s milk, or a formula based on cow’s milk, at a level higher than could be explained by a mother’s diet.

Online breast milk sales are a small but growing market. Five times as many ads for the service, or 55,000, were posted in 2014 compared with 2011, the Wall Street Journal reports.

Only the Breast (OTB) does recommend best practices for both buyers and sellers of breast milk on the site but has no mechanism in place to ensure the quality of milk sold through the site. “We believe most OTB donors are honest, abiding by OTB terms and are simply looking to provide safe milk for babies in need,” the company said in a statement in response to the study, as the Seattle Times reports.

The study isn’t the first to raise concerns about the legitimacy and safety of the online breast milk market. Earlier research by the same group found that more than 75 percent of breast milk samples ordered online were contaminated with bacteria.

“Added cow’s milk is just the tip of the iceberg,” Keim writes. “There is no way for parents to know what else is in milk purchased online.”

Keim points out that these results probably do not apply to donated milk that is exchanged through nonprofit milk banks because many facilities screen donations and there is no financial incentive for donors to stretch the product. However, donations to some banks have declined as new parents realize they can make extra money by selling milk online instead. Mother’s Milk Bank in San Jose, Calif., for instance, gave out 500,000 ounces of donated milk in 2014, down from 570,000 ounces in 2013, as San Francisco Gate reports.