The drug has been linked to overdoses across the Unites States John Moore/ Getty Images

A drug so powerful it is used to sedate elephants is contributing to hundreds of heroin-related overdoses in the US.

Carfentanyl is a potent tranquiliser, around 10,000 times more powerful than morphine. This week, heroin laced with the substance – so toxic it has been compared to nerve gas – is suspected to have played a part in the deaths of two men in Eagle County, Colorado.

"Carfentanyl, which has been linked to a significant number of overdose deaths in various parts of the country, has been located here in our community," the Eagle County Sheriff's Office stated.

Capsules containing heroin laced with the substance were discovered at the scene.

Over the last few months, several fatal overdoses linked to carfentanyl have been reported. The drug has been found mixed with heroin in Ohio, West Virginia, Florida and Kentucky.

Between August and September 2016, around 300 people across four states in the US overdosed on heroin linked to carfentanyl and fentanyl – a less powerful but similar substance.

The Drug Enforcement Administration states carfentanyl is around 100 times more powerful than fentanyl, which is 50 times more powerful than heroin.

Prescription pain addiction is a problem prevalent in economically depressed areas of the US.

What is carfentanyl and how powerful is it?

The drug is a powerful synthetic opioid in the same class as heroin and fentanyl, normally used to sedate large animals such as elephants. The drug is so toxic that it has been banned from conflict zones under the Chemical Weapons Convention.

The side effects of carfentanyl are similar to those of fentanyl. They include itching, nausea and respiratory depression, which can be life-threatening. carfentanil is much stronger than fentanyl – which has been linked to a spate of deaths in the US.

The problem garnered attention after police in Ohio released a graphic image of an overdosed couple unresponsive in their car while a four-year-old boy sat in the back.

According to the DEA, carfentanyl comes in different forms including tablets, spray and blotter paper.

The Associated Press reported that carfentanyl, fentanyl and other opioids are actively marketed by Chinese chemical companies.

In China, carfentanyl is not a controlled substance and is manufactured legally and sold over the internet.