Abnormally Slow Heart Rate Linked to Xylazine-Fentanyl Overdose Raises New Concerns for Emergency Care
Researchers at the Icahn School of Medicine at Mount Sinai have identified bradycardia, or an abnormally slow heart rate, as a key clinical sign associated with overdoses involving xylazine mixed with illicit opioids, particularly fentanyl. This finding adds an important new layer to how emergency physicians may recognize and respond to overdoses involving this increasingly common drug combination, especially in regions where xylazine has become widespread.
Xylazine is not an opioid. It is a powerful veterinary sedative and pain reliever approved only for use in animals. Yet in recent years, it has quietly become a frequent adulterant in the illicit fentanyl supply across the United States. Because it is added without users’ knowledge, people who use opioids are often exposed to xylazine unintentionally, creating medical risks that are still not fully understood.
One of the biggest challenges for clinicians is that xylazine cannot be detected at the bedside in emergency departments. Unlike many opioids, it does not show up on standard rapid drug screens. Confirming its presence requires a blood test sent to a specialized toxicology laboratory, which means doctors often have to make treatment decisions without knowing whether xylazine is involved. The new research suggests that observing a markedly low heart rate may help fill this gap.
How the Study Was Conducted
The Mount Sinai research team carried out a prospective observational study involving 1,289 adult patients who were treated for suspected opioid overdose. These patients were seen in emergency departments across 10 health care institutions in the United States over a three-year period, from September 2020 through September 2023.
Blood samples collected during routine clinical care were de-identified and later sent to a specialized forensic toxicology lab. Researchers tested these samples for xylazine and a range of other substances. Patients were then divided into two groups: those with both xylazine and an illicit opioid detected in their blood, and those with only an illicit opioid present.
The researchers analyzed vital signs recorded when patients arrived at the emergency department, including heart rate and blood pressure. They also examined demographic factors such as age, gender, race, and geographic location to see whether any patterns emerged.
What the Researchers Found
Out of the 1,289 patients studied, xylazine was detected in 238 individuals. Among those patients, 6 percent arrived at the emergency department with bradycardia, compared to only 2 percent of patients who did not have xylazine detected. This difference was statistically significant.
In practical terms, patients experiencing a xylazine-fentanyl overdose were about twice as likely to have an abnormally slow heart rate as those overdosing on opioids alone. Importantly, bradycardia was the only vital sign that showed a strong and meaningful association with xylazine exposure in this group.
The study did not identify other consistent clinical markers that could reliably signal xylazine involvement. That makes the heart-rate finding particularly valuable, as it is based on a routine, easily measured vital sign that clinicians already monitor.
Strong Regional Patterns in Xylazine Exposure
Geography played a major role in the findings. Approximately 75 percent of patients with xylazine detected were treated in the Northeastern United States. Compared to this region, the likelihood of detecting xylazine was:
- 60 percent lower in the Midwest
- 70 percent lower on the West Coast
- 97 percent lower in the Southeast
These numbers reinforce what public health officials have been observing for several years: xylazine has become especially entrenched in the Northeast’s illicit fentanyl supply, even as it continues to spread to other parts of the country.
Why Bradycardia Matters in the Emergency Department
In emergency medicine, every minute matters. When a patient arrives after an overdose, doctors must quickly decide how to stabilize them and what risks they may face. Because naloxone only reverses opioid effects, it does not counteract xylazine’s sedative or cardiovascular impact.
Recognizing bradycardia as a possible sign of xylazine exposure gives clinicians a new tool. Even without confirmatory lab results, they may be able to suspect xylazine involvement and adjust care accordingly. This can include closer cardiovascular monitoring, supportive treatment, and discussions with patients about harm-reduction strategies once they recover.
Understanding Xylazine and Its Effects
Xylazine acts as an alpha-2 adrenergic agonist, a class of drugs known to suppress the central nervous system. In animals, it produces sedation, muscle relaxation, and pain relief. In humans, especially when combined with opioids, it can lead to deep sedation, low blood pressure, slowed breathing, hypothermia, and cardiac abnormalities.
One of the most visible long-term effects linked to xylazine exposure is the development of severe skin lesions and wounds, sometimes occurring far from injection sites. These wounds can become infected and are often difficult to heal.
Despite its growing presence in the illicit drug supply, xylazine remains poorly understood in humans, largely because it was never approved for medical use in people. This creates significant knowledge gaps for clinicians and researchers alike.
Challenges in Detection and Treatment
A major issue highlighted by the study is the lack of rapid testing options for xylazine. Emergency physicians cannot currently confirm exposure in real time, making it difficult to tailor treatment or fully explain risks to patients.
Additionally, because xylazine is not an opioid, standard overdose response protocols are often incomplete. Naloxone remains essential and lifesaving for opioid overdoses, but it does not reverse xylazine’s effects. Patients may regain breathing but remain deeply sedated or unstable due to the non-opioid component.
What This Means Going Forward
The study helps lay the groundwork for identifying a potential xylazine-related toxidrome, or recognizable cluster of symptoms. While bradycardia alone is not enough to diagnose xylazine exposure, it provides an important clinical clue in the absence of immediate lab confirmation.
Researchers are now expanding their work to explore whether xylazine exposure is linked to outcomes such as length of hospital stay, intensive care unit admission, and how blood concentrations of xylazine correlate with vital signs like heart rate and blood pressure.
As xylazine continues to spread beyond the Northeast, findings like these may become increasingly relevant for emergency departments nationwide.
Research Reference
Clinical factors linked to xylazine exposure in emergency department patients with illicit opioid overdose, Addiction (2026)
https://onlinelibrary.wiley.com/doi/10.1111/add.16500
