Detection, pharmacokinetics and cardiac effects following administration of clenbuterol to exercised horses.
Authors: Knych H K, Mitchell M M, Steinmetz S J, McKemie D S
Journal: Equine veterinary journal
Summary
# Editorial Summary: Clenbuterol Detection and Withdrawal Times in Racing Thoroughbreds Clenbuterol's use in performance horses demands clear withdrawal protocols to prevent inadvertent doping violations, yet evidence-based guidance has been lacking for the dosing regimens commonly employed in practice. Researchers administered clenbuterol to 22 racing-fit Thoroughbreds via a chronic low-dose protocol (0.8 µg/kg twice daily for 30 days) and to 6 horses using the manufacturer's escalating dose schedule, then tracked plasma and urine concentrations using liquid chromatography-mass spectrometry for up to 35 days post-treatment. Plasma clenbuterol became undetectable (below 10 pg/ml) by day 4 in the low-dose group and day 7 in most horses receiving escalating doses, whilst urine concentrations persisted longer—dropping below the limit of quantification between days 21 and 28 across both groups. Although some horses exhibited transient muscle fasciculations, sweating and elevated heart rates following administration, these effects resolved rapidly, indicating quick tolerance development. These pharmacokinetic data provide racing jurisdictions with the baseline evidence needed to establish realistic withdrawal periods tailored to their detection thresholds, helping practitioners advise clients on safe administration windows and mitigating the risk of positive findings from therapeutic use.
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Practical Takeaways
- •If clenbuterol is used therapeutically in racing horses, withdrawal times must be established based on urine detection thresholds (21-28 days) rather than plasma clearance (4-7 days) to prevent positive regulatory findings
- •Acute side effects from clenbuterol administration (muscle fasciculations, sweating, tachycardia) are generally mild, transient, and horses develop tolerance rapidly
- •Racing jurisdiction regulations should clarify which detection method (plasma vs. urine) and threshold they use, as this directly impacts safe withdrawal periods before competition
Key Findings
- •Plasma clenbuterol concentrations fell below the limit of quantification (10 pg/ml) by Day 4 in the chronic low-dose regimen (0.8 μg/kg twice daily for 30 days) and by Day 7 in most horses receiving escalating doses
- •Urine clenbuterol remained detectable for 21-28 days post-administration in the low-dose group, extending detection windows beyond plasma clearance
- •Transient adverse effects including muscle fasciculations, sweating, and increased heart rate occurred in a small number of horses but resolved rapidly with tolerance development
- •Study provides pharmacokinetic data for two commonly used dosing regimens to establish appropriate withdrawal times for racing jurisdictions