"We found that one in 10 samples of milk that we purchased online had high levels of cow's milk or infant formula added to it.
"When people are selling their milk online they may have an incentive financially to top off their milk with cow's milk or baby formula to increase the volume and make more money."
That can be dangerous for moms who are turning to breast milk online because their baby has a dairy allergy.
"They can have a serious allergic reaction or have a lot of gastrointestinal illness when they drink cow's milk," Keim said. "I think it's really buyer beware. You don't know what's in the milk that you are purchasing and I think it's clear from the series of studies we've conducted, that milk purchased on the internet is not safe for babies."
Researchers recommend moms talk to their pediatrician to decide what feeding plan is best.
"Every baby is different and I think it's really important for parents to work closely with their pediatrician to consider the babies health," Keim said.