Fentanyl is a synthetic opioid, 50 times more powerful than heroin. Carfentanil is 100 times stronger than fentanyl.

Carfentanil, which is also known as Wildnil, has an analgesic potency that's 10,000 times stronger than morphine and 100 times more potent than fentanyl, according to the US Drug Enforcement Administration. It's often used to tranquilize large animals, like horses. In several states, law enforcement has found carfentanil popping up in overdose cases where the drug was mixed into heroin or disguised as heroin. In addition to being mixed with heroin, carfentanil is also being sold in pill form. The CDC noted that carfentanil was created in 1974 and isn't approved for human use because of how powerful it is.

Excerpt from Pharmacy Times:
https://www.pharmacytimes.com/news/7-things-to-know-about-carfentanil

Carfentanil is a synthetic fentanyl analog. It is a mu-opioid receptor agonist with an estimated analgesic potency approximately 10,000 times that of morphine and 20-30 times that of fentanyl, based on animal studies. Its extreme potency and propensity to produce rapid and profound respiratory depression has prompted recommendations that an opioid antagonist, such as naloxone or naltrexone, be available whenever carfentanil is used or suspected to be present. Carfentanil (Wildnil) has been used in veterinary as a prescription-only general anesthetic for intramuscular injection in large animals. Carfentanil is no longer FDA-approved for use in animals after Wildlife Laboratories withdrew the application for Wildnil. Carfentanyl is increasingly involved in opioid overdose deaths among illicit opioid users.

Carfentanil:
https://drugs.ncats.io/drug/LA9DTA2L8F

Carfentanil was first synthesized by a team of chemists at Janssen Pharmaceutica in 1974. Today, it is marketed under the trade name Wildnil, and is most often used as a general anesthetic agent for large animals, such as elephants and bears. Due to its extreme potency, carfentanil is intended for large-animal use only and is inappropriate for use in humans.

the life of every person who is a street drug consumer is in danger because of this new mix. Anyone buying heroin off the streets must be wary of the fact that it may be combined with carfentanil, which has the highest potential for fatality.

Similar to the administration of fentanyl through transdermal patches, carfentanil can also be easily absorbed through the skin, as well as inhaled. This poses an additional, grave risk to first responders and law enforcement on the scene of an overdose, who need to use personal protective equipment when handling carfentanil in emergency situations. It may also affect family members, friends, or just a passerby who has the potential to accidentally touch or inhale carfentanil in the vicinity.

Excerpt from Sober Nation:
https://sobernation.com/deadly-carfentanil/

World Health Organization 2017:
Carfentanil, a synthetic opioid agonist,isconsidered to beone of the most potent opioids known. It was developed by Janssen Pharmaceuticain 1974 and is an analog of the opioid analgesic fentanyl. It is estimated to be approximately 10,000 times more potentas an analgesic than morphine. Carfentanil is a controlled compound in 19countries (Austria, Australia, Belgium, Canada, China, Cyprus, Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Ireland, Latvia, Lithuania, Poland, Sweden, the United Kingdom, and the United States)and is used primarily as a tranquilizer in large animals. It is not intended for therapeutic use in humans,although it has been used widely in the scientific community as a radio tracer in positron emission tomography (PET)imaging studies in both humans and laboratory animals.

Its extreme potency and propensity to produce rapid and profound respiratory depression has prompted recommendations that an opioid antagonist, such as naloxone or naltrexone, be available whenever carfentanil is used or suspected to be present. No controlled laboratory studies have evaluated the abuse potential of carfentanil in any species.However, several reports of illicit carfentanil seizures have appeared around the world over the past few years. It is typically added to or sold as heroin or prescription pills on the illicit market, often unbeknownst to the user. Reports of carfentanil-laced cocaine are also now appearing in the United States. Reports of fatal overdoses involving carfentanil are increasing substantially, in part due to the availability of methods to detect carfentanil in body fluids. Carfentanil poses a serious threat to the public health.

World Health Organization Carfentanil Critical Review Report (PDF 27 pages):
https://www.who.int/medicines/access/controlled-substances/Critical_Review_Carfentanil.pdf

EMCDDA - Europol Joint Report 2017:
Carfentanil has been available in the European Union since at least December 2012. Since 2016, there has been an increase in the number of seizures reported. Typically it is seized as a powder, including in mixtures with heroin and other illicit opioids. To date, it has been detected in 8 Member States and Norway.

The most serious acute health risk posed by carfentanil is likely to be rapid and severe respiratory depression, which can be life-threatening. The antidote naloxone can reverse acute poisoning, including respiratory depression.

In total, 48 deaths with confirmed exposure to carfentanil have been reported by six Member States and Norway. Most of the deaths have occurred in the last six months.

Carfentanil is sold and used as a substitute for illicit opioids and prescription opioids. In at least three Member States, carfentanil has been sold as or in heroin. Similar to other fentanils, serious concerns exist that the substance could be supplied surreptitiously to a range of drug users. Most users are unlikely to be aware that they are using carfentanil.

From the available information in the European Union, it does not appear that carfentanil has been granted a marketing authorisation as a medicinal product for human or veterinary use.

European Monitoring Centre for Drugs and Drug Addiction Carfentanil Joint Reports (PDF 19 pages):
http://www.emcdda.europa.eu/system/files/publications/6502/2017.6256_EN_04-WEB.pdf

DEA Officer Safety Alert Carfentanil (PDF 1 page):
https://www.dea.gov/sites/default/files/divisions/hq/2016/hq092216_attach.pdf

Carfentanil:

Carfentanil is legally controlled in most jurisdictions, but has veterinary uses for anaesthetising large animals, such as elephants and bears.

• A structural analog of the synthetic opioid analgesic fentanyl
• First synthesized in 1974
• Was sold starting in 1986 under the brand name "Wildnil" for use in tranquilizer darts
• Commercial production of Wildnil ceased in 2003, and the drug is available only as a compounded dosage form
• For pain relief, a unit of carfentanil is 100 times as potent as the same amount of fentanyl, 5,000 times as potent as a unit of heroin and 10,000 times as potent as a unit of morphine.

• Health professionals are increasingly concerned about the potential escalation of public health consequences of its recreational use
• Carfentanil is most often taken with heroin or by users who believe they are taking heroin.
• Carfentanil is added to or sold as heroin because it is less expensive, easier to obtain and easier to make than heroin.

The effects of carfentanil, including overdose, can be reversed by naloxone