There's a lucrative online market for buyers of human breast milk, but this craze poses serious health risks.

Although there are websites and forums claiming their breast milk are "clean" and can help with erectile dysfunction and cancer, researchers led by Dr, Sarah Steele of the Global Health and Policy Unit at Queen Mary University of London found that these purported benefits do not stand up clinically.

They say raw human milk purchased online or in an unpasteurised state poses many risks.

"Nutritionally there is less protein in breast milk than other milks like cow's milk," Steele said. "Potential buyers should be made aware that no scientific study evidences that direct adult consumption of human milk for medicinal properties offers anything more than a placebo effect."

She and her colleagues warn that human milk is potentially very hazardous if used to replace a healthy balanced diet.

They also note that the failure of women to sanitize properly when expressing milk, the failure to sterilise equipment properly, and the improper or prolonged storage and transportation of milk can expose consumers to bacterial food-borne illnesses like any other raw milk.

The lack of pasteurisation and testing not only indicates a bacterial risk but also exposes consumers to a host of infectious diseases, including hepatitis, HIV and syphilis.

"While many online mums claim they have been tested for viruses during pregnancy, many do not realise that serological screening needs to be undertaken regularly," Steele said. "Sexual and other activities in the postpartum period may expose the woman expressing to viruses that they may unwittingly pass on to consumers of the milk."

Researchers call for health professionals and regulators to issue public guidance against the purchasing of human milk from Internet sources for adult as well as infant feeding.

The findings are detailed in the Journal of Royal Society of Medicine.