The tranquiliser for wild game that is
the most powerful commercially available opioid.

Simon Cotton
University of Birmingham

Molecule of the Month December 2018
Also available: HTML version.

Why is an elephant tranquiliser being found in heroin?

Paul JanssenIs that anything like fentanyl?

Yes, fentanyl (MOTM March 2015), the parent drug of the family, was originally made in Belgium in 1960 by a team at Janssen Pharmaceutica, led by Dr Paul Janssen (photo, right), who was trying to devise better analgesics with fewer side effects and higher safety margins. They were looking for molecules that were more lipid-soluble, so they would get into the central nervous system faster and so give more rapid analgesic action; other derivatives followed, including carfentanil in 1974. However, carfentanil is a good 100 times stronger than fentanyl. And fentanyl is 100 times stronger than morphine, which makes carfentanil the most potent commercially available opioid. In fact, it is so toxic that it has no approved use in humans.

Fentanyl Carfentanil
Fentanyl Carfentanil

Why is it so much stronger than fentanyl and morphine?

Introducing a sidechain into a drug molecule often leads to a big change in its strength. Just over half a century ago, pharmaceutical chemists tweaked the morphine molecule (MOTM Nov 2004) and produced etorphine (MOTM May 2002). Etorphine is around 1000 times as potent as morphine, with an LD50 estimated to be around 30 μg for humans. Such small changes in molecules must make them ‘fit’ the body’s receptor better, and so increase their potency.

What use is a powerful medicine like carfentanil?

Like etorphine, carfentanil has been widely used by vets in particular to anaesthetise large animals like elephants, rhinos or ‘big cats’; it is the active ingredient in Wildnil. No one knows the lethal dose of carfentanil to humans, but it must be similar to that of etorphine, or less, in the region of 20 μg.

How do vets use it?

Well, animals don’t roll up their sleeves ready for the injection; the vets use ‘darts’ containing a solution of the desired sedative, the darts are essentially syringes fitted with a hypodermic needle and are shot out of a gun by compressed gas. The darts are stabilised by a ‘flight’, a tuft of fibre, which makes it fly straight at the animal.

Tranquiliser darts A lion shot with two tranquilliser darts
A vet shooting an elephant in India

Top left: Tranquilliser darts filled with carfentanil.

Top right: A lion shot with two tranquilliser darts
before being fitted with a GPS tracking device.

Bottom left: A vet shooting an elephant in India
with a tranqilliser dart after it ran amok in the street.

That makes it sound very simple.

It isn't. Because carfentanil (or etorphine) is so toxic, it presents a danger to any human using it, so quite a few precautions must be in place. When carfentanil is used by vets – from preparing the darts until after their use - they must work in groups of at least two, and wear whole-face covering as well as latex gloves, plus clothing that covers the whole body (i.e. not T-shirts and shorts). Several vials of the antidote naloxone (MOTM November 2018) have to be carried (one is not enough). There is even a case for using a ‘glove box’ – like an infant incubator fitted with latex gloves – for the process of making up the darts and loading the gun. And, of course, needles and syringes must not be thrown away but disposed of safely after use.

This sounds a bit extreme, are these precautions really needed?

Yes, as people can get injected accidentally, whilst carfentanil can get introduced into the body by contact with broken skin or a cut, as well as though mucous membranes, like the eyes, or through inhaling dust. Carfentanil is so toxic that some people worry that it could be absorbed through the skin.

In one case reported in 2010, a vet was trying to sedate a wild elk, and had to remove a dart from a tree-trunk, in the process getting splashed in the face, eyes and mouth. Despite immediately washing his face with water, he started to feel drowsy within a couple of minutes, and his colleagues had to administer naltrexone (the ‘reversal agent’ used with animals, as naloxone was not available) whereupon he largely recovered by the time he arrived at A&E an hour later.

OK, but these are extreme cases, surely there is no problem?

Sadly not. The painkiller crisis in the USA has mushroomed since oxycontin (MOTM Dec 2015) arrived on the market in the mid-1990s, but fentanyl abuse goes further back, to the late 1970s, when it was one of the drugs, along with high-purity heroin, known on the streets as ‘China White’. Today, ‘China White’ can mean heroin cut with fentanyl. It has become a big problem in the 21st century, but carfentanil only seems to have become a drug of abuse since around 2016. In the latter part of that year carfentanil was identified in around 400 seizures by police. Most of these were in Ohio, centred upon Cincinnati. Subsequently cases have cropped up in Minneapolis, Chicago, New York, and other centres. Carfentanil has also been detected in blood samples from ‘under-the-influence’ drivers. It’s been reported that from 1 September 2016 to 1 January 2017, carfentanil was identified in 262 post-mortem blood specimens in the USA. Carfentanil has even reached the UK.

Opioid overdose deaths in the USA 2015

Where’s it coming from?

One day in June 2016 a bunch of Mounties (Royal Canadian Mounted Police officers) wearing respirators, facemasks and hazmat suits, with tape covering the joins, swooped on a consignment from China of printer accessories at the airport in Vancouver. It said ‘printer accessories’ on the box, but the blue cartridges labelled as ink for HP printers contained a kilogram of carfentanil. The police arrested a man in Calgary for whom the order was destined.

Mounties swooping on a consignment from China of printer accessories at the airport in Vancouver

So China’s the source?

Certainly one of them, it seems. They used to refer to inhaling the vapour from hot solution heroin or opium as ‘chasing the dragon’. Carfentanil makes you think of the movie ‘Enter the Dragon’, with Bruce Lee. A reason for this is that in the USA fentanyl and derivatives like carfentanil have long been Schedule II drugs, whilst drugs like these were unregulated in China until 2016. After that, the DEA persuaded the Chinese authorities to act and regulations came into force in March 2017. Of course, ‘underground’ laboratories can continue to supply illicit drugs via the ‘Dark Web’, operating from fake addresses.

Leading to…

Users may decide that mixing a fentanyl derivative with heroin will give them a more intense ‘rush’, but for many the consequence has been death. Other addicts have been unaware that their heroin has been cut with a more toxic drug, and have received a lethal overdose in consequence. People have also simply taken accidental overdoses of fentanyl thinking they were milder painkillers, and died as a result. For example, in April 2016 the pop star Prince died when he took fentanyl-based painkillers thinking they were the much milder Vicodin). In addition, the singer Tom Petty died in October 2017 from accidentally taking an overdose of a mixture of painkillers including fentanyl, despropionylfentanyl and acetylfentanyl.

Prince Tom Petty
Prince Tom Petty

And it was used in the Moscow theatre siege?

On October 23rd 2002, 40 Chechen terrorists occupied the Dubrovka Theatre in Moscow, taking around 900 people hostage. The siege was brought to a close on October 26th, when Russian authorities pumped a ‘chemical agent’ into the building’s ventilation system. The terrorists were killed by Russian special forces, but around 130 of the hostages were killed by the chemical, whose identity was not known. Subsequent analyses on urine and clothing samples from survivors showed it to contain carfentanil.

Unconscious liberated hostages are taken away in a bus Russian special forces turned to carfentanil to end a standoff
Unconscious liberated hostages are taken away in a bus from the scene near the Dubrovka Theatre in Moscow.
Russian special forces turned to carfentanil to end a standoff with Chechen separatists
using an aerosol version of the opioid.
(Photo: Dmitry Lovetsky/The Associated Press).

Remifentanyl was also identified.



Can you sum up carfentanil in a few words?

How about the phrase used by Lady Caroline Lamb to describe George Gordon, Lord Byron (1788-1824)?

Which was?

Mad, bad, and dangerous to know!


It’s the elephant tranquiliser in the room.


Fentanyl and carfentanil

Carfentanil as a veterinary sedative and its use

Safe handling of carfentanil

Carfentanil and the Moscow theatre siege

Deaths of Prince and Tom Petty

Carfentanil binding site

The opioid and fentanyl crisis

Fentanyl detection

counter Back to Molecule of the Month page.        [DOI:10.6084/m9.figshare.6139097]