Prevention of drug residues in the beef and dairy industries is a major concern as both meat and milk from cattle are widely consumed by humans around the world. This paper will address the keys to avoiding residues; however, there are several things that can change the half-life of a drug and thus affect the withdrawal time. These factors can include route of drug administration, volume administered at each injection site, drug formulation, and disease. The focus of this presentation will be to discuss what is known about disease induced alterations in the pharmacokinetics of drugs and how clearance time in many cases may be delayed resulting in residue violations. Withdrawal times are generally based on pharmacokinetic studies done in healthy animals; however, there is strong evidence that these times may not always be appropriate in cows with clinical disease. Since pharmaceutical companies must conduct trials to demonstrate the efficacy of various drugs for treating a specific disease or condition during the approval process, it seems logical that pharmacokinetic and residue studies could be done using the same animals or under similar conditions.