Over-the-Counter Antihistamines & Heat Stress

Allergic rhinitis (AR) currently affects \~25% of Canadians, and due to factors of climate change, this number is expected to increase over the coming decade. AR symptoms can significantly impact individuals' quality of life by compromising sleep, productivity, and social interactions. To alleviate AR symptoms, North Americans tend to rely on H1 antihistamine medications available over-the-counter (OTC) at most pharmacies. However, public health authorities currently suggest restraining all antihistamines during heat waves due to beliefs that M3 muscarinic receptor and H1 receptor antagonism, independent pharmacological mechanisms of H1 antihistamines, might suppress thermoregulatory responses to heat stress and increase individuals' susceptibility to heat-related illness/injury. To date, studies using supramaximal doses of antihistamines have demonstrated reductions in sweating, however these doses and administration routes are not the typical use case. Additional studies utilizing fexofenadine, a second-generation H1 antihistamine, have linked H1 receptor antagonism to reductions in skin blood flow, potentially impacting thermoregulation by reducing peripheral blood redistribution. Empirical evidence supporting OTC H1 antihistamines impacting thermoregulatory control at recommended doses is scarce. Thus, this study aims to systematically assess whether three common OTC H1 antihistamines, taken as prescribed, alter thermoregulatory responses during thermal stress.