Thermal and Cardiovascular Responses during Exertional Heat Stress after Diphenhydramine Use: A Randomized Crossover Trial

Introduction: Despite sparse systematic evidence, current exercise heat safety recommendations suggest that antihistamines blunt sweating and increase the risk for heat-related injury during exertional heat stress. The primary purpose of the present study was to examine whether diphenhydramine hydrochloride (DPH), a first-generation antihistamine, affects the sweating, core temperature, and heart rate (HR) response during exertional heat stress using a double-blind randomized crossover design. Methods: On two occasions separated by >48 h, 20 healthy adults (10 females, 23 +/- 3 yr, body surface area: 1.9 +/- 0.2 m (2) , body mass index: 23.7 +/- 2.2 kg center dot m (-2) ) orally consumed either 50 mg of DPH or placebo (PLA), and then rested for 2 h in a climate-controlled room maintained at 30 degrees C and 35% relative humidity (heat index of similar to 29 degrees C), followed by a 60-min fixed-heat production treadmill walk (6.3 +/- 1.0 W center dot kg (-1) ). Whole-body sweat loss, local sweat rate, rectal temperature ( T-rec ), and HR were measured. Results: Whole-body sweat loss was not different between conditions (PLA: 406 +/- 78 g, DPH: 396 +/- 75 g; P = 0.26, treatment effect: -10 g; 95% confidence interval, -28 to 8). No differences were observed for the onset of sweating (PLA: 13.5 +/- 2.4 min, DPH: 13.3 +/- 2.7 min; P = 0.79) and steady-state local sweat rate (PLA: 0.83 +/- 0.26 mg center dot cm (-2) center dot min (-1) , DPH: 0.82 +/- 0.27 mg center dot cm (-2) center dot min (-1) ; P = 0.99). No difference in baseline T-rec was observed (PLA: 37.09 degrees C +/- 0.35 degrees C, DPH: 37.13 degrees C +/- 0.33 degrees C; P = 0.68), and the 60-min Delta T-rec was not different ( P = 0.99) between PLA (0.83 degrees C +/- 0.29 degrees C) and DPH (0.81 degrees C +/- 0.30 degrees C). HR was similar at baseline (PLA: 86 +/- 13 bpm, DPH: 84 +/- 11 bpm; P = 0.30) and end-exercise (PLA: 134 +/- 28 bpm, DPH: 132 +/- 26 bpm; P = 0.90). Conclusions:<bold> </bold>Fifty milligrams of DPH does not modify the sweating, core temperature, and HR response during exertional heat stress in young healthy adults.