The impact of long-term clenbuterol on athletic performance in horses.

The research discusses the effects of long-term use of clenbuterol, a drug used to manage recurrent airway obstruction (RAO) in horses. Counter to potential misuse by competitors for performance-enhancement, the study indicates that prolonged administration of clenbuterol may actually reduce aerobic capacity in horses and have other potential side effects.

Background

• Recurrent Airway Obstruction (RAO), a common respiratory disease in horses in temperate regions, is caused by hypersensitivity to mold spores in the stable environment. The best way to manage it is through a minimum dust management regime that reduces the animal’s exposure to allergens.
• When environmental control can’t be achieved, pharmaceutical support is usually required. Clenbuterol, a beta-2 sympathomimetic agent, has been used for about 30 years to treat horses with RAO, mainly to relieve bronchospasm. It also has been found to increase the mucociliary clearance rate and may have anti-inflammatory effects.
• Clenbuterol, when administered at the lowest recommended dosage, had its effectiveness questioned as only 25% of horses reportedly responded. Higher doses were found to increase the drug’s effectiveness, but adverse effects resulting from its beta-1 sympathomimetic activity, such as sweating and excitement, were also increased.

Adverse Effects of Clenbuterol

• The authors discuss the reported adverse effects of clenbuterol in horses and other species. Long-term administration of the drug, even at 2.4 µg/kg, has potential significant side effects.
• Competitors hoping to enhance the horse’s performance through the anabolic, repartitioning effects of clenbuterol will discover that, contrary to improving athletic performance, prolonged clenbuterol administration actually reduces aerobic capacity in clinically-normal horses.
• The authors further reveal that clenbuterol can negatively impact maximum oxygen uptake, muscle physiology, and capacity for high-intensity exercise. It also adversely affects other body systems, for example, endocrine and reproductive functions.
• Clenbuterol also has cardiovascular effects following acute exercise in horses given the drug long term, specifically increasing the aortic root diameter.

Recommendations

• Veterinarians prescribing clenbuterol need to be aware of the drug’s potential adverse effects as very few pharmaceuticals are entirely risk-free. More work is needed to fully understand the degree of risk associated with clenbuterol in a clinical setting.
• Clinical experience indicates that short-term use of clenbuterol at the lower end of the accepted dose range poses relatively low risk. The biggest potential danger associated with the short-term use of clenbuterol in RAO-affected horses is possibly the owners failing to alter their horse’s management that is crucial for successfully controlling this disease.