Eligible treatment-seeking , cocaine-dependent subjects entered a 4-week baseline period ( Phase I ) with high-value abstinence contingent vouchers and two motivational interviewing sessions , followed by a 12-week medication trial ( Phase II ) with random assignment stratified on Phase I abstinence status to ( 1 ) modafinil ( 400mg/d ) , ( 2 ) levodopa/carbidopa ( 800/200mg/d ) , ( 3 ) naltrexone ( 50mg/d ) , or ( 4 ) placebo .