Consider this:

A 10 milligram dose of the animal tranquilizer carfentanil is powerful enough to sedate, even kill, a 15,000-pound African elephant, and more than strong enough to take down a musk ox, bull moose or fully grown buffalo.

If diluted sufficiently, a dose of the same size — just the fraction of the weight of a paper clip — could also send 500 humans to the morgue.

So when authorities say they’re worried about kilograms’ worth of the drug making their way into the North American heroin supply, they mean it.

Carfentanil is the most potent commercial opioid in the world — 10,000 times stronger than morphine — and law enforcement from northwest Canada to the eastern U.S. have raised alarms about it tainting heroin batches, and even being sold as the drug itself.

Heroin cut with carfentanil offers a harder-hitting, longer-lasting high and allows dealers a shortcut to increase their supplies. But users often don’t know what they’re getting. In recent months, authorities have linked carfentanil to a spike in overdoses in several states, and have warned that it could spread to others.

The emergence of carfentanil poses a new set of problems for drug enforcement officers, who are already spread thin fighting a nationwide epidemic of opioid addiction. In addition to heroin, authorities have struggled to contain the spread of fentanyl, a more powerful heroin cousin that has killed thousands of users in recent years — perhaps most famously Prince in April 2016.

Now, carfentanil is on the radar too.

“You feel like a kid with his finger in the dike, you know?” Joseph Pinjuh, a Department of Justice drug task force chief based in Ohio, told the Associated Press. “We’re running out of fingers.”

Just this week, Canadian law enforcement agencies announced charges against a man they said was supposed to be on the receiving end of a one kilogram package of carfentanil shipped from China earlier in the summer.

In a news conference Tuesday, the Canadian Border Services Agency and the Royal Canadian Mounted Police said they found the drug hidden in a box labeled as printer accessories and addressed to a man in Calgary. They charged the man, age 24, with one count of importing a controlled substance and one count of possession for the purpose of trafficking.

The package, authorities said, could have produced 50 million doses.

“It is hard to imagine what the impact could have been if even the smallest amounts of this drug were to have made its way to the street,” Canadian Border Services Regional Director Roslyn H. MacVicar said in a statement.

In the United States, Ohio has been ground zero for the drug’s appearance. Last month, prosecutors indicted a Columbus man they claim passed off a batch of the drug as heroin, causing 10 overdoses, one of them fatal. Rayshon Alexander has pleaded not guilty to 20 counts, including murder and drug trafficking. Investigators initially thought the deaths were caused by fentanyl, but lab results came back positive for carfentanil, prosecutors said.

Also in July, police in Akron, Ohio, reported that paramedics had logged 236 drug overdoses in a three-week period — nearly as many as they’d seen in all of the first half of the year, according to the Akron Beacon Journal. Police in the city’s suburbs reported similar trends, the Beacon Journal reported.

“Without toxicology reports, it’s hard to know for sure,” said Lt. Rick Edwards of the Akron Police Department, “but it’s looking like this is because of carfentanil.”

The rise in overdoses has prompted Ohio Attorney General Mike DeWine to urge police to stop field testing drugs on the scene, out of fear that they could be handling substances much more powerful than they realize.

“It’s just too high of a risk,” DeWine said, according to the Columbus Dispatch. “This stuff is just now hitting. You’re really not seeing [police] departments with any experience with it at all.”

On Thursday, prosecutors in western Pennsylvania issued a similar warning to law enforcement, saying more than 200 recent overdoses in the region — 20 of them fatal — may be linked to carfentanil. The drug has also been blamed for spates of overdoses this year in Kentucky and Florida.

It’s hard to overstate just how deadly carfentanil is.

For starters, carfentanil, in liquid form, is odorless and colorless, making it virtually impossible for recreational drug users to know what they’re taking, or how much. They may think they’re getting heroin or fentanyl, but whatever they shoot or ingest could contain the exponentially stronger analog.

Veterinarians who handle the drug wear protective gloves, aprons and masks, treating it “almost like uranium,” in the words of one zoo veterinarian who spoke to Fusion. A dose the size of a grain of salt could kill a person, and carfentanil can even be lethal when absorbed through the skin, according to the Drug Enforcement Administration.

The anti-overdose drug naloxone can be used to treat a carfentanil overdose, but it takes a lot. One or two shots is usually enough to counteract a heroin overdose, but a half-dozen or more are required for carfentanil.

To put carfentanil’s strength in a different perspective, the Buffalo Field Campaign, which works to protect bison in Yellowstone National Park, says the drug is so powerful that humans shouldn’t even eat the meat of animals sedated with it.

Carfentanil may have even been used as a chemical weapon that caused the deaths of 170 people in a hostage crisis in Russia in 2002. When Chechen separatists seized a movie theater in Moscow, authorities deployed an unknown gas through the building’s ventilation system and raided it. The 40 attackers died, along with 130 of the hostages. A 2012 analysis of survivors’ urine and clothing by British scientists showed that carfentanil was part of the chemical agent pumped through the theater.

Brian Escamilla, a California forensic chemist formerly with the DEA, told the Calgary Herald that the kilogram of carfentanil recently seized in Canada was jaw-dropping. He said authorities everywhere face an uphill battle detecting it because only a minuscule amount is required to dramatically boost the effects of a hit of heroin.

“There’d be a hell of a lot more heroin in there than there would be carfentanil, and so the carfentanil would just be a blip on the screen,” Escamilla told the Herald. “Whether the toxicologists pick that up or not … that could easily be missed.”