The relative amounts of heroin, fentanyl and carfentanyl you need to take to overdose.

A drug up to 5,000 times more potent than heroin has reached the shores of Singapore.

The controlled drug carfentanyl, also known as carfentanil or its street name CF, is 5,000 times more potent than a unit of heroin, and 10,000 times more potent than a unit of morphine.

It is derived from fentanyl, which is also a controlled drug about 100 times more potent than a unit of morphine.

According to a news release by the National Crime Agency (NCA) in the UK, an organised crime group shipped packages of the lethal drugs to Singapore sometime between December 2016 and April last year.

The group had marketed the drugs on the dark web, and mailed orders from a post office in Leeds, England, to more than 400 buyers around the world, including Singapore.

It is not known who the drugs were mailed to here.

There have been no prior cases involving the drugs reported in the media here.

According to the NCA, Jake Levene, 22, Lee Childs, 45, and Mandy Christopher Lowther, 21, sold the drugs on the AlphaBay dark web market under the name UKBargins.

The drugs were marketed as "dangerous and lethal", but also of at least 98 per cent purity.

The listings have since been taken down.

Mr Greg McKenna, regional head of investigations at the NCA, called the trio a "main supplier" of the synthetic opioids.

"There have been more than 120 UK deaths relating to fentanyl or carfentanyl since December 2016," he said. "They knew exactly how lethal the drugs were but continued to sell them."

It was previously reported that the drugs are a growing threat worldwide. (See report, right.)

A spokesman for the Central Narcotics Bureau (CNB) said the drugs are illegal here.

"Fentanyl and carfentanyl are listed as Class A controlled drugs under the Misuse of Drugs Act," he said.

"Unauthorised consumption, possession, trafficking, import or export of any controlled drug is an offence under the law."

While fentanyl is used here in small doses as a painkiller, carfentanyl has no medical use for humans, but is sometimes used on large animals, like elephants.

Carfentanyl has also been described as a potential chemical weapon, comparable to nerve gas.

The trio were arrested in April last year, and 677g of pure carfentanyl was found in their drugs facility. The amount seized was the equivalent to "millions of lethal doses".

It was also revealed by the NCA that Lowther went into a coma in February last year from the drugs and was hospitalised, but recovered and continued to supply the drugs. The trio will be sentenced in the Leeds Crown Court next month.