Nitazenes

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Nitazenes are a family of drugs derived from a core benzimidazole structure. They were created in the late 1950s by the Swiss company Ciba AG as potential pain relievers. Like other opioids, they act on the central mu-opioid receptor, but they are extremely potent. Only a few nitazenes are as strong as fentanyl, and etonitazene and isotonitazene can be even more potent.

Because of serious side effects, nitazenes were never approved for medical use in humans or animals. Since 2019, highly potent nitazenes have appeared on the illegal drug market in North America and Europe as new synthetic opioids, contributing to the opioid crisis. There have been many overdose deaths linked to these drugs.

The story begins with desnitazene, a related compound with some pain-relieving effect. Through systematic changes to the molecule, researchers found more powerful nitazenes by adding nitro groups and other substitutions. Etonitazene, one of these compounds, is among the strongest nitazenes known.

Their pain-relieving action is similar to morphine, as shown by the fact that its effects can be blocked by other opioid drugs. In a 1958 clinical trial involving etonitazene and clonitazene with 363 patients, researchers found a very narrow therapeutic index, meaning the safe dose is very close to the harmful dose. This made them impractical as medicines.

In the 2000s and 2010s, nitazenes began to appear more in the drug scene. Isotonitazene was the first to be sold on the darknet and has been found in overdose deaths in Switzerland and Canada. The drugs have fueled problems in the United Kingdom and the Baltic states, with hundreds of overdose deaths reported up to early 2025.

Chemically, nitazenes are benzimidazoles with a dialkylaminoethyl group at the 1-position and a substituted benzyl group at the 2-position; the 5-position often has a nitro group. Substitutions at other positions affect potency. The para position on the benzyl group is especially important for activity. Some modifications to the methylene linker are tolerated, but they can reduce activity.

Nitazenes are structurally different from most other opioids, though they share similarities with certain benzimidazole derivatives from related drug families. Since 2024, immunoassay-based test strips have been available to help detect nitazenes, but they have limitations. The tests are most likely to miss other closely related nitazene drugs, and some nitazenes may not be detected.

In humans, nitazenes are metabolized in ways that depend on the species. The main human pathways are N-deethylation and, for 4'-ethers, O-dealkylation. 4'-hydroxy metabolites are cleared quickly in urine. Enzymes such as CYP3A4 and CYP2C8 are involved in N-deethylation. The nitro group can be reduced outside the liver by gut bacteria. Other metabolites, like N3-oxide, also form. The 4'-hydroxy compounds in urine and the N-deethyl compounds in blood are useful clues for forensic analysis.