Factors affecting the activity of carboxylesterases would be expected to markedly alter the pharmacokinetics and clinical effects of substrate drugs. One key factor that could affect catalytic activity is drug interactions that inhibit carboxylesterase function. The importance of inhibition of drug metabolism in medication safety and efficacy is well established for drugs that undergo metabolism by cytochrome P450 enzymes. In distinct contrast, little is known about the potential for carboxylesterases to serve as a target for metabolic inhibition mediated by drug interactions.