People are dying and doctors can stop it.
Americans just experienced the hottest year in U.S. history, and it’s only getting worse. Climate change is making summers hotter, longer, and deadlier. While everyone feels the heat, people who use drugs are facing some of the gravest, most overlooked consequences.
The link between hot weather and substance use mortality isn't new, but it has now been quantified. In 2010, researchers looking at substance use in New York City noticed a pattern: overdose deaths spiked on very hot days, and dropped when the weather was cool. As summers became hotter, places like Arizona and British Columbia observed similar, disturbing connections with overdose deaths.
Now, based on over two decades of data, scientists from Yale University have confirmed heat as the driver. Every 1°C (about 2°F) increase in the heat index creates a measurable, predictable increase in overdose mortality from use of opioids, cocaine, and methamphetamine, alone or in combination.
This shouldn’t come as a surprise. Heat is already associated with higher mortality for a wide range of chronic conditions like cardiovascular disease and diabetes that are often comorbid with substance use. About half of people with mental health disorders have also been diagnosed with substance use disorder, and vice versa, and even small increases in ambient temperature are linked to increased substance use from psychological distress. People who experience homelessness, mental illness, and also use substances face the extra threat of constant outdoor exposure.
Substances also act directly to increase the risk of mortality in hot weather. The physiologic response to heat is controlled by the hypothalamic-pituitary-adrenal (HPA) axis, which activates vasodilation, sweating, and thirst responses. Each substance disrupts or amplifies this response in different ways.
Opioids directly bind to receptors in the hypothalamus, depressing the HPA axis and the body’s physiologic response to heat. Other side effects, such as hyperhidrosis, blunted thirst sensation, and decreased perception of heat can lead to profound dehydration and low drive to seek cooler environments. Heat also disrupts dopaminergic pathways and increases opioid receptor affinity, leading people to report needing higher doses to achieve the same drug effect.
Conversely, stimulants like cocaine and methamphetamine activate the HPA axis, directly increasing core body temperature while simultaneously masking the body's natural discomfort signals. Vasoconstriction offsets natural vasodilation responses and compounds cardiac demand. "Overamping" — the clinical term for a stimulant overdose — occurs more frequently and at much lower doses when the weather climbs above 24°C (about 75°F), leading to extreme hyperthermia and cardiac strain.
When substances are used together, the risks only multiply.
Doctors can help. All heat-related deaths are preventable, but we rarely talk to our patients about prevention.
Remind patients who may use drugs to do so in temperature-controlled environments, since outdoor use increases in the summer. Help them plan for emergencies by sharing mail delivery harm reduction resources for when high temperatures make it hard to travel, or identifying opioid treatment alternatives when power outages close clinics. Simple interventions can save lives, such as informing patients that cars can become dangerously hot in minutes, or helping them locate friendly, local cool spaces, like drug-user health hubs and air-conditioned syringe centers.
Our approaches to diagnosis need to evolve. Heat-related illness shares many overlapping symptoms with withdrawal, and heat stroke can easily be misattributed to overdose, or vice versa. Clinicians should routinely take a core body temperature for patients presenting with a suspected overdose in the summer months.
Physicians need to inform patients how extreme heat affects their bodies. Educate people who use drugs on the signs and symptoms of heat exhaustion and heat stroke. Stress that their tolerance decreases in extreme heat and so they should use at lower doses, less frequently, and never alone in hot weather. Advise patients to hydrate regularly even if they do not feel thirsty, and remind them to store opioid reversal agents like Narcan in a temperature-controlled place rather than a glove box. Share patient heat toolkits in your clinic or hospital.
Substance use disorder is not going away, and neither is climate change. As our environment shifts, the way all of us treat patients who use drugs must adapt. Patients who use drugs engage with every part of our healthcare system, from ERs to specialty offices to primary care clinics. Each of these visits is an opportunity. Clinicians have the tools to prevent these heat-driven deaths. It’s time to start using them.
