Testing shows it can contain potentially dangerous bacteria.
A first-of-its-kind study out of Nationwide Children's Hospital in Ohio shows up to 75-percent of breast milk bought on one particular website had high amounts of bacteria, including salmonella, that could make babies sick.
Human breast milk for babies is typically sold on the internet for a few dollars an ounce.
The researchers in this case bought and tested about 100 samples.
They did not identify which milk-sharing website they used.
Three out of four of their samples had either high levels of bacteria growth overall or had disease-causing bacteria, including fecal contamination.
The study also cites several cases where babies got sick from a stranger's breast milk.
The researchers highlight several reasons for this. They say the results are probably due to poor hygiene during milk collection or shipment.
That means the milk-sharer likely used unclean containers or unsanitary breast milk pump parts.
They found nearly 20-percent of sellers also did not include dry ice or other cooling methods when shipping the milk.
If you need to purchase breast milk for your baby, you may want to look into a milk bank. Banks tend to screen donors and pasteurize the donated milk to kill germs.