Deaths caused by a synthetic opioid 500 times stronger than heroin have likely been underestimated by up to a third, researchers suggest.
Nitazenes are a class of synthetic opioids that are extremely potent and can be manufactured at a low cost. The drugs were originally manufactured as a painkiller in the 1950s, but their development was halted due to their extreme potencies resulting in a high risk of addiction.
The presence of nitazenes on the unregulated drug market has risen steeply in the last seven years – prompting UK and international bodies to issue public health warnings about their use.
While the National Crime Agency (NCA) reported 333 fatalities linked to nitazenes in 2024, researchers at King’s College London believe that the number of deaths has been underreported, as concerns have been raised by toxicologists regarding their stability in postmortem blood samples. This means nitazenes are likely being missed by postmortem toxicology tests.
Dr Caroline Copeland, senior lecturer in pharmacology and toxicology at King’s College London, said: “If nitazenes are degrading in post-mortem blood samples, then we are almost certainly undercounting the true number of deaths that they are causing. That means we’re trying to tackle a crisis using incomplete data.
“When we don’t measure a problem properly, we don’t design the right interventions – and the inevitable consequence is that preventable deaths will continue.”
The study by King’s College London researchers, published on Monday in Clinical Toxicology, sheds light on the UK’s growing synthetic opioid problem.
Researchers tested samples from rats that had been anaesthetised with the drug and found, on average, only 14 per cent of nitazene present at the time of overdose was present when tested under real-world pathology and toxicology sample handling conditions.
The academics then applied modelling to data from the UK National Programme on Substance Use Mortality (NPSUM) to reveal there was an excess of drug deaths in Birmingham in 2023 by a third (33 per cent).
Because it typically takes around a month for blood samples to be analysed by toxicologists, researchers believe a credible explanation for at least some of these excess deaths may be due to the non-detection of nitazene that degraded prior to toxicology testing being performed.
“This research shows that the harm caused by nitazenes is likely being significantly underestimated. Because these drugs degrade in post-mortem blood, we may be missing up to a third of the deaths they are involved in, meaning public health responses are being designed and funded for only two-thirds of the real problem,” Dr Copeland said.
“Behind this undercount are people dying suddenly from extremely potent opioids, families left without answers, and communities facing a growing but largely hidden toll.”
Synthetic opioids such as fentanyl, methadone, and buprenorphine have been linked to 524 deaths in England since the end of September 2025, according to the Department of Health and Social Care (DHSC).
The government has provided £310 million, in addition to the public health grant, to improve drug and alcohol treatment services and recovery support in England, in a bid to drive down drug use.
Naloxone – a fast-acting medication designed to reverse opioid overdoses – is also being rolled out to 32 more police forces.
A government spokesperson said: “Every death from the misuse of drugs is a tragedy.
“This government is committed to reducing drug-related deaths and supporting more people into recovery to live healthier, longer lives. We remain on high alert to emerging drug threats, working closely with health services and policing partners to stay ahead of criminals who target our communities.
“Border Force has also become the first agency in the world to use specially trained dogs to detect both deadly fentanyl and nitazenes.”
