The WHO warned some fake injections may not contain semaglutide at all or others may contain other medications, such as insulin, leading to “unpredictable” effects.
“These falsified products could have harmful effects to people’s health,” the WHO said.
The UK’s drugs regulator seized a batch of fake Ozempic in October last year.
They had come into the country from legitimate suppliers in Austria and Germany, and appeared to have authentic packaging.
Dr Alison Cave, the organisation’s chief safety officer, said: “Buying semaglutide from illegally trading online suppliers significantly increases the risk of getting a product which is either falsified or not licensed for use in the UK.”
She said injecting such products “may put your health at risk”.
A higher strength version of Ozempic has been developed specifically for weight-loss and is marketed as Wegovy.
This is being offered at specialist weight-loss clinics on the NHS in the UK.