In the U.S., Rising Temperatures May Increase Overdose Deaths

Across the United States, rising temperatures due to climate change pose a growing threat to public health. Extreme heat exposure has been linked to increases in premature deaths, cardiovascular and respiratory diseases, suicide rates, and violent crime. A growing body of research additionally points to a connection between heat and fatal drug overdoses.

A new study led by researchers at Yale School of Public Health (YSPH) and Yale School of Medicine (YSM) and published in Addiction on October 6, 2025, investigated the association between heat exposure and drug overdose deaths on a national scale.

People who use drugs may be especially vulnerable to the adverse health effects of heat exposure. Some drug use can directly increase body temperature. Drug use can also impair an individual’s ability to recognize and respond to overheating. For example, respiratory depression associated with opioid use can disrupt the body’s compensatory efforts to cool down. Furthermore, drug use can compound adverse effects of overheating. Both heat exposure and stimulant use, for instance, are independently associated with cardiovascular problems. Stimulant use in combination with heat exposure can exacerbate cardiovascular risk.

In the Addiction study, Julia Dennett, PhD, and colleagues examined county-level mortality data across the continental U.S. in June through September of each year from 1999 to 2020. The researchers employed a novel strategy to compare counties against each other year-to-year and assess fluctuations in heat and effects on overdose. Dennett, the study’s lead author, was a postdoctoral researcher with YSPH when the study was conducted. She is currently a postdoctoral researcher with the Hasso Plattner institute in Potsdam, Germany. Other co-authors on the study were Dr. Gregg Gonsalves (senior author) and Dr. Daniel Carrión from YSPH, and Dr. David Fiellin, MD, from YSM.

To appropriately measure heat exposure and account for associated health risks, the researchers employed the monthly average maximum heat index—which considers both temperatures and relative humidity—and considered other relevant variables in their analysis such as precipitation and air pollution levels.

The researchers found that heat exposure was associated with increased drug overdose deaths, regardless of drug type, during the study period. In the hottest months of each year, there were 150 excess deaths annually. The greatest number of deaths occurred in recent years as temperatures continue to rise. Consistent with the broader literature, heat exposure appeared to impact overdose deaths involving stimulants such as cocaine and methamphetamine. Heat exposure also was associated with increased opioid overdose deaths. Larger effects in overdose mortality were observed after 2013, when fentanyl began to dominate the illicit opioid supply.

“We observed an association between heat and all drug overdose deaths, including deaths related to opioids, cocaine, and methamphetamines. The time period we studied experienced both rising temperatures and an evolving drug supply, and the relationship between heat and drug overdose deaths reflects these ongoing forces,” said Dennett.

The increasing prevalence of polysubstance use, and co-use of opioids and stimulants in particular, further complicates overdose risk in the setting of extreme heat. Other factors such as income, housing status, urban environment, and use of certain psychiatric medications, can also shape individuals’ relative risk for heat exposure as well as their ability to thermoregulate. Dennett and colleagues ran additional analyses accounting for some of these factors, finding that urban and suburban counties and counties with greater levels of social vulnerability as measured by the Social Vulnerability Index, saw the greatest impact of heat exposure on overdose death rates.

“Effects were largest in counties with more social vulnerability and in urban and suburban areas. Social determinants of health—including income, housing, and the local environment— are critical to understanding and addressing the risks of heat exposure for people who use drugs,” said Dennett.

As extreme heat events become more frequent in the U.S., Dennett and colleagues call on policymakers to address the heightened risk people who use drugs face amidst the ongoing, intersecting climate and overdose crises.

“There are many factors that make individuals more susceptible to the impacts of heat. A wider recognition that drug use is involved in heat vulnerability may allow us to use data to tailor and target interventions,” said Daniel Carrión, PhD, MPH, assistant professor of epidemiology (environmental health) at YSPH and a co-author of the study. “This study puts two seemingly distinct public health crises —the overdose crisis and the climate crisis—in conversation. Recognizing that extreme weather can compound the risks of drug-related overdose is an important step towards effective public health efforts.”

The authors recommend improving public health surveillance infrastructure to track heat and overdose and, in so doing, empower data-driven interventions. They also encourage implementation of heat exposure mitigation strategies such as expansion of public vegetation and green spaces, provision of low-barrier housing, assistance with water and electricity utilities for low-income households, and installation of cooling stations. They additionally highlight a need for targeted education to raise awareness of the risks of heat exposure, particularly in the context of drug use, and promote strategies that may reduce individual risk of overdose and other adverse health outcomes.

The research reported in this news article was supported by grant funding from the National Institute on Drug Abuse (Dennett and Gonsalves, DP2DA049282; Gonsalves, R37DA15612). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Drug Abuse.