PURPOSE: This study investigated pharmacokinetics of terbutaline after single and seven consecutive days of inhalation in exercising trained men.

METHODS: Twelve healthy young trained men underwent two pharmacokinetic trials comparing single dose (2 mg) and seven consecutive days (2 mg×d) of inhaled terbutaline. After inhalation of terbutaline at each trial, subjects performed 90 min of bike ergometer exercise at 65% of maximal oxygen consumption after which they stayed inactive. Blood and urine samples were collected before and after inhalation of terbutaline. Samples were analyzed by high performance liquid chromatography-tandem mass spectrometry.

RESULTS: Maximum serum concentration of terbutaline (Cmax) (6.4 (±1.2) vs. 4.9 (±1.2) ng×mL, P = 0.01) (mean (±95% confidence interval)) and area under serum concentration-time curve from 0 to 4 hours after inhalation (AUC0-4) (16 (±3) vs. 13 (±2) ng×mL×h, P ≤ 0.005) were higher after seven days of inhalation compared to the first day. Seven days of terbutaline inhalation resulted in accumulation of terbutaline in urine, in which total urine excretion of terbutaline was higher after seven days of inhalation compared to the first day (274 ± (43) vs. 194 (±33) μg, P ≤ 0.001). These differences were partly attributed to systemic accumulation of terbutaline after consecutive days of inhalation, in that baseline serum and urine samples revealed incomplete elimination of terbutaline.

CONCLUSION: Terbutaline accumulates in serum and urine after consecutive days of inhalation. For doping control purposes, these observations are of relevance if a urine threshold and decision limit is to be introduced for terbutaline on the World Anti-Doping Agency's list of prohibited substances, since asthmatic athletes may use their bronchorelievers for consecutive days.