Australian Border Force officials have issued a bleak warning about dangerous drug W-18.
The street drug is considered even more potent than fentanyl, the powerful opioid that killed singer Prince and was found at the house where Michael Jackson died.
Even a tiny amount of fentanyl can be fatal, making it attractive to drug dealers worldwide.
Several seizures of W-18 have been made at the Australian border, The Daily Telegraph reports.
But W-18 is not an opiate. It was developed in the early 1980s by Canadian researchers as an analgesic and non-addictive painkiller.
It is available in both pill and powder form, and cannot be detected in the bloodstream.
Many who take the drug are likely to believe they are taking heroin, adjunct research fellow with the National Drug Research Institute Dr Stephen Bright told news.com.au.
"That's the real danger, because they're likely to die if they think it's heroin.
"W-18 is just one of hundreds of these new opiates and other drugs that are being designed to be used as counterfeits. In the last 12 months, we've seen as significant increase in the drugs being sold as heroin.
"If you go back 20 years, everything was more clear - heroin was heroin, MDMA was MDMA and so on."
He says carfentanyl, 100 times stronger than fentanyl, is being sold on the streets in the United States.
"It's used on animals. It's used to take down elephants. The amount of the reversal agent required for humans is six times that of a heroin overdose."