Hidden sellers promote unlicensed weight-loss drugs through WhatsApp and Telegram competitions.
They offer injectable medicines such as retatrutide as prizes.
The Guardian observed groups using countdowns and giveaways to attract users.
Experts warn these tactics pose serious health risks.
Retatrutide remains experimental and lacks approval anywhere in the world.
Other prizes include unapproved peptides such as melanotan II.
UK law allows weight-loss injections only with valid prescriptions.
Unlicensed drugs cannot be marketed or sold legally.
Some sellers disguise drug sales as fitness or coaching programmes.
Researchers say these practices undermine safety and medical oversight.
