Dutch authorities are warning people not to order prescription painkillers online after a 30-year-old man died in Amsterdam after taking fake oxycodone.

According to the Trimbos Institute, the Dutch research agency, the pills were bought online without a prescription, Euronews reported.

They did not contain oxycodone - which is prescribed to patients in severe pain - but nitazenes, which are synthetic opioids much stronger than oxycodone, morphine or fentanyl.

According to Trimbos, the drugs are so strong that just a few milligrams can cause fatal breathing problems. The effect also lasts longer, increasing the risk of overdose.

Police in Amsterdam are still trying to establish where the deceased man got hold of the fake oxycodone. But his death is one of several health crises linked to synthetic drugs in the Netherlands recently.

The Dutch Inspectorate for Health and Youth Care (IGJ) said multiple people received emergency medical treatment after taking pills that were sold online as oxycodone but turned out to contain nitacenes.

The drugs were sold online and on social media, the inspectorate said. They look like oxycodone and usually have original packaging - making them "almost indistinguishable" from the real thing.

"This shows how dangerous it is to order prescription drugs online," IGJ chief inspector Henk de Groot said.

"They may look real, but you have absolutely no idea what you're buying," de Groot added.

The warning comes amid an increase in the availability of nitacenes and other synthetic opioids in parts of Europe, according to the EU drugs agency, which described them as an "important emerging threat".

"Trimbos urged people to avoid buying painkillers online and said people should call for emergency help if they start to feel unwell after taking oxycodone. | BGNES