Residues of clenbuterol in cattle receiving therapeutic doses: implications for differentiating between legal and illegal use.

Clenbuterol (CBL) can be used legally in the treatment of respiratory diseases and illegally as a growth promoter in animals. Liver and eye have previously been shown to be effective matrices for the detection of residual concentrations of the drug. The pharmacokinetics of CBL in therapeutically treated cattle were investigated. During treatment, many body fluids and tissues contained residues of the drug. After a 14 day withdrawal only eyes (mean 27.1 micrograms/kg), and to a much lesser extent lung and kidney (mean 0.3 micrograms/kg), contained detectable residues. By day 28 of withdrawal only residues in eyes were present (mean, 6 micrograms/kg). These persisted to the end of the trial (42 days withdrawal). It is concluded that it is not possible to differentiate between the legal and illegal use of CBL solely on drug residue analysis. Other information must be made available to regulatory bodies to enforce control programmes.