To characterize the practice of breast milk sharing via the internet in the US and examine factors associated with participants’ communication regarding potential health and safety risks. This cross-sectional study examined all original postings (n = 254) placed during 1 week in 2011 on four websites to facilitate the sharing of breast milk. Postings were characterized for intent and health and safety topics (i.e., selling vs. donating milk, hygiene/handling practices, infectious disease screening, diet/exercise habits, substance and pharmaceutical use, milk quality claims, price) communicated between milk providers and recipients. Approximately 69 % of postings were providing milk and 31 % were seeking milk; 47 % included identifiers. Few provider postings reflected measures to potentially reduce risks to recipients: 20 % mentioned using a healthy handling/hygiene practice, 11 % offered specifics about infectious disease screening, 51 % mentioned limiting/abstaining from 1+ substances. The presence of indications about handling/hygiene, diet/exercise, and abstaining from substances were strongly positively associated with each other (ORs 7.42–13.80), with the odds of selling (ORs 6.03–∞), and with making quality claims (ORs 3.14–13.54), but not with disease screening. One-fifth of recipients sought milk for a child with a medical condition or poor birth outcome. Most recipients (90 %) did not specify any health and safety practices of a provider in their posting. Health behaviors and screening for diseases that may affect milk safety are not prominent topics in postings seeking to share milk. This lack of communication may exacerbate the health risks to recipient infants, especially infants at increased risk due to pre-existing health conditions.