This double-blind, placebo-controlled trial will occur in 3 parts. In Part 1, the individual pharmacokinetics of an intravenous dose of cocaethylene will be determined in order to estimate individualized cocaethylene infusion rates and pharmacokinetic parameters. This will provide important information on how cocaethylene is processed by the body. In Part 2, an infusion of cocaethylene, producing a venous plasma concentration of 200 ng/ml, will be administered over an 8-hour period. Clinical monitoring and blood sampling will occur in order to determine the safety profile of cocaethylene. During Part 3, the ability of cocaethylene to modify the acute effects of intravenous cocaine will be determined. Cocaethylene will be administered in plasma concentrations of 0 ng/ml, 50 ng/ml, or 200 ng/ml over an 8-hour period. Participants will be randomly assigned to receive a challenge intravenous dose of cocaine (0.5 mg/kg or 1 mg/kg) or placebo at the 4-hour mark of cocaethylene infusion. Following the initial 8-hour period, cocaethylene infusion will be continued for an additional 5 hours. Behavioral and physiological measures will be collected throughout the study sessions at predetermined times to evaluate whether tolerance to cocaethylene develops. These measures will also help to determine whether cocaethylene modifies or produces tolerance to the effects of an acute dose of cocaine.