Grey-market injectable peptides – a category of substances with obscure, alphanumeric names such as BPC-157, GHK-Cu, or TB-500 – have developed a devoted following among biohackers and health optimisers. To understand how these unregulated substances have become mainstream and what they could be doing in our bodies, Madeleine Finlay hears from journalist Adrienne Matei and from Dr Anna Barnard, an associate professor at Imperial College London who researches peptides