The Dutch health ministry and addiction clinic Trimbos have issued a warning about a life-threatening fake version of the drug oxycodone which has already killed one user in the Netherlands.

The fake pills contain a different chemical –  isotonitazene – a synthetic opioid which, according to the British medical journal The Lancet, is 250 times more potent than heroin.

The person who died last Friday had taken the fake drug, having bought the pills online without a prescription. Oxycodone is a highly addictive painkiller used in the Netherlands by cancer patients and as part of palliative care.

Isotonitazene and other nitazenes “work for longer than other strong synthetic opioids and that increases the risk of overdosing,” Trimbos said. “A couple of milligrammes can cause serious health risks such as breathing difficulties, which in turn can lead to death.”

Another version of the drug, protonitazene, was found earlier by researchers for the Dutch Drugs Information and Monitoring System in fake oxycodone pills which had been bought online.

Chief health inspector Henk de Groot said the incident “shows how dangerous it is to buy pills which require a prescription online.”

“It might look real, but you have absolutely no idea what you are buying,” he said.