"Breast is best". So goes the message from the international and clinical guidance on what milk mothers should feed their babies. But it's also more worryingly been adopted by a growing online community of adults wanting to buy and consume expressed breast milk for its perceived health benefits – or due to sexual fetishes.

Some online forums suggest cancer patients should drink breast milk because it is supposedly easier to digest, better tolerated, and full of immune benefits, including immunoglobulin (a protein used by the immune system). Meanwhile, fitness and diet forums preach the nutritional, energy or recovery benefits of such milk, suggesting it can work as a supplement to workout or bulking regimes.

A number of websites and online forums cater to those wishing to buy, sell and trade breast milk, alongside the use of more general social media platforms. This online marketplace allows women who are expressing milk to advertise with text and images, communicating details such as cost per ounce and a description of mother, milk and baby. Buyers can also advertise on such forums, detailing their own needs and volume requirements.

Individuals can then contact each other either to meet or arrange transport for the milk, which is often frozen or packed in dry ice, and shipped by express post or courier. Notably, the quality of packaging greatly varies, and studies have shown high levels of damage in transit.

The popularity of these sites varies by country depending on the availability of government-subsidized milk banks. But in the US, where regulated milk banks are costly, and the UK, where adult buyers are not catered for, online selling communities have been growing. New country-specific websites are now being launched, including using .co.uk addresses. Such growth has led commentators to label online breast milk sale a "booming market" around the world.