Cocaine dependence remains a serious public health problem; however no clearly effective pharmacological treatments have been identified to date. The investigators hypothesize that identification of subgroups of cocaine-dependent patients will help to develop targeted and more effective treatments. The investigators have observed that 30-40% of cocaine-dependent patients who enter our medication trials achieve abstinence during the lead-in period (the two weeks prior to starting medication). Initial abstinence is strongly predictive of abstinence during the subsequent medication trial. The investigators have also observed that a low dopamine release in the striatum is associated with greater choice of cocaine in volunteers and failure of cocaine-dependent patients to respond to behavioral treatment. The investigators hypothesize that individuals who have difficulties in achieving abstinence have a deficit in dopaminergic functioning and correcting this deficit using dopaminergic medication LCE (levodopa in combination with carbidopa and entacapone) will result in clinical improvement.