People taking antihypertensives showed a greater increase in heat-related illness during heatwaves (IRR: 1.64) compared to when not taking antihypertensives (IRR: 1.42), with evidence of drug-temperature interaction, suggesting antihypertensive users are particularly vulnerable to heat-related illness during heatwaves.
Key Findings
Results
The rate of heat-related illness during heatwaves was greater among person-time prescribed antihypertensives than among person-time not prescribed antihypertensives.
IRR for heat-related illness during heatwaves versus baseline when prescribed antihypertensives: 1.64 (95% CI: 1.53–1.75)
IRR for heat-related illness during heatwaves versus baseline when not prescribed antihypertensives: 1.42 (95% CI: 1.27–1.58)
Evidence of drug-temperature interaction was statistically significant (p < 0.01)
The study used a self-controlled case series (SCCS) design, comparing within-individual rates during heatwave periods versus baseline periods
Results
ACE inhibitors/ARBs were associated with an increased risk of heat-related illness during heatwaves.
The finding was identified in subgroup analyses examining individual antihypertensive drug classes
The pattern of increased heat-related illness during heatwaves was consistent for ACE inhibitors/ARBs
This class of medication affects the renin-angiotensin system, which can influence fluid balance and temperature regulation
Results
Calcium channel blockers (CCBs) were associated with an increased risk of heat-related illness during heatwaves.
The increased risk pattern during heatwaves was consistent for CCBs in subgroup analyses
CCBs affect vascular tone, which may influence thermoregulation
This finding was consistent with the overall pattern observed for the combined antihypertensive group
Results
Non-loop diuretics did not show evidence of an effect on heat-related illness risk during heatwaves.
Subgroup analysis for non-loop diuretics did not demonstrate a consistent increase in heat-related illness during heatwaves
This contrasted with findings for ACE inhibitors/ARBs and CCBs
Non-loop diuretics were one of three antihypertensive classes examined alongside ACE inhibitors/ARBs and CCBs
Methods
The study used a self-controlled case series design with administrative claims data from adults with hypertension in Tsukuba City, Japan, over approximately five years.
Data were drawn from Tsukuba City administrative claims data between April 2014 and March 2019
Average daily temperature was derived from Japan Meteorological Agency data
Heatwave periods were defined with 5-day pre-heatwave and 5-day post-heatwave windows compared to baseline
All models were adjusted for age and season
The SCCS design compares rates within individuals, inherently controlling for time-invariant confounders
Methods
The study compared heat-related illness rates not only during heatwaves but also during 5-day pre-heatwave and 5-day post-heatwave periods relative to baseline.
The analysis examined incidence rate ratios for heat-related illness during pre-heatwave (5-day), heatwave, and post-heatwave (5-day) periods
Baseline periods were defined as non-heatwave, non-periwave periods within the same individuals
This temporal structure allowed assessment of the immediate and surrounding period risk associated with heatwaves
What This Means
This research suggests that people taking blood pressure medications (antihypertensives) face a greater increase in heat-related illness during heatwaves compared to periods when they are not taking these medications. Using health insurance claims data from Tsukuba City, Japan, collected between 2014 and 2019, the researchers compared each person's rate of heat-related illness during hot weather to their own rate during normal temperatures. This within-person comparison helps rule out the possibility that the results are simply due to sicker or older people being more likely to both take medications and get heat-related illness. The study found that heat-related illness rates rose about 64% during heatwaves among people taking antihypertensives, compared to about 42% among the same people during periods when they were not on these medications.
When the researchers looked at specific types of blood pressure medications, they found that ACE inhibitors, angiotensin receptor blockers (ARBs), and calcium channel blockers were each associated with this increased vulnerability to heat during heatwaves. However, non-loop diuretics (another common blood pressure drug type) did not show the same pattern. These medication classes work through different biological mechanisms — some affect blood vessel tone and others affect how the kidneys handle fluid — which may explain why they have different relationships with heat risk.
This research suggests that the approximately one billion people worldwide who take antihypertensive medications may be a group that needs particular attention during heatwaves as climate change makes extreme heat events more frequent and intense. However, the authors caution that it is not yet clear whether the medications themselves cause the increased risk or whether other differences between medicated and unmedicated periods (such as underlying disease severity) explain the finding. More research is needed before conclusions can be drawn about whether medication management during heatwaves should change.
Harley M, Iwagami M, Kawamura C, Williamson E, Rentsch C, Mathur R, et al.. (2026). Antihypertensive Medications and Risk of Heat-Related Illness During Heatwaves: Self-Controlled Case Series.. Pharmacoepidemiology and drug safety. https://doi.org/10.1002/pds.70447