Drug users are taking new psychoactive substances (NPS) — otherwise known as legal highs — without knowing the specific contents, the UN Office on Drugs and Crime (UNODC) said.
Others are buying drugs believing they are more common substances such as ecstasy or MDMA without realising they are actually a new designer drug, the UNODC added in its report on synthetic drugs.
Emergency services are then unable to identify substances taken by users when they fall ill and cannot provide appropriate treatment.
Synthetic drugs researcher Natascha Eichinger said: "The danger here is people are consuming new psychoactive substances without knowing that they're doing so. So once these people have severe health issues and they wish to receive treatment they don't actually know what they've taken."
The report, the 2014 Global Synthetic Drugs Assessment, added: "Emergency services may therefore find themselves unable to identify life-threatening substances and powerless to administer the proper treatment to users."
Elsewhere, the report said 348 so-called legal highs have been identified in more than 90 countries since 2008, although this number could be significantly higher as the figure only reflects reports of official sources and does not take unofficial sources into account.
Some 97 substances were identified in the last year alone.
The UNODC warns that designer drugs have gained popularity, particularly among the young, and are no long restricted to "niche" markets.