A little-known opioid that’s deadlier than fentanyl has led to a growing number of overdose deaths in recent years.

Nitazenes, a class of synthetic opioids, were originally developed as pain relievers in the 1950s, but were never approved for use due to the high overdose risk, according to a report from the Inter-American Drug Abuse Control Commission (CICAD).

Starting in 2019, nitazenes (benzimidazole-opioids) began to enter the illicit drug market in Europe, where they have been reported on almost every continent.

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CICAD’s report suggests that "nitazene use is a growing trend in North America and that availability is likely spreading across the Americas."

The most prevalent type of nitazene is isotonitazene (ISO), but at least a dozen others have been identified, the report stated.

The U.S. Drug Enforcement Administration (DEA) classifies ISO as a Schedule 1 drug, along with seven other nitazenes.

"Nitazenes are being produced in illicit labs overseas, mostly in places like China and India, and are getting trafficked into the U.S. through the same channels used for fentanyl," Corey Gamberg, a licensed alcohol and drug counselor and executive director at the Massachusetts Center for Addiction, told Fox News Digital. 

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Because they’re synthetic and cheap to make, they’ve become a popular option for illegal manufacturers, the expert noted.

"As authorities crack down on fentanyl, traffickers pivot to new synthetic opioids like nitazenes to get around enforcement," Gamberg went on. 

"These drugs are cheap to make and easy to move. It's a pattern we’ve seen before, and it’s repeating itself now."

Nitazenes are available in pills, powders, sprays and other forms. They can be snorted, smoked, injected or taken orally.

The biggest risk is potency, as some types of nitazenes are several times stronger than fentanyl.

Dr. Jonathan Avery, vice chair of addiction psychiatry at NewYork-Presbyterian/Weill Cornell Medical Center, confirmed that nitazenes can be hundreds of times stronger than morphine and up to 10 times more potent than fentanyl, "making even tiny doses deadly."

In addition to potentially fatal overdoses, the drugs are known to cause dependency, dizziness, nausea, vomiting, disorientation, loss of consciousness, cardiac arrest and seizures, according to the CICAD report.

The other issue is that people usually don’t know they’re taking them. 

"Nitazenes are often mixed into heroin, pressed pills or even stimulants — so someone might think they’re using one thing and end up taking something completely different," Gamberg cautioned. "That unpredictability is what makes them so deadly."

Adding to the danger of nitazenes is that testing is very limited, according to Dr. Ryan Marino, a medical toxicologist at Case Western University in Cleveland, Ohio.

"Test strips are available that can help people screen things like powdered drugs and pills for the presence of nitazenes, but more definitive testing is limited to more advanced toxicology labs, and also limited by cost and turnaround time," he told Fox News Digital.

"While test strips are useful, they are not a definitive way to rule out nitazenes in drugs, and more advanced testing for nitazenes is not widely available, even in post-mortem testing."

Routine hospital and workplace drug screens do not reliably detect nitazenes, according to Avery.

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"Enforcement is difficult because producers constantly tweak chemical structures to evade laws and standard tests," he said.

To prevent deadly overdoses, Avery called for specialized testing and rapid drug‑checking tools to identify nitazenes in the street supply.

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"Naloxone (Narcan) can reverse nitazene overdoses, but multiple doses may be needed because of their potency," he said.

"Public awareness, widespread naloxone distribution and rapid drug‑checking are key to preventing deaths."