Abstract
L-tryptophan (ie, tryptophan) has shown promise as a pharmacotherapy in cocaine addiction treatment. Abstinent contingent voucher incentives have shown efficacy in abstinence initiation and maintenance for treatment of cocaine dependence. The present study evaluated these two approaches singly and in combination in a relapse prevention + treatment design. A double-blind, parallel-group, placebo-controlled design was used. Cocaine-dependent patients (N = 199) were stratified and randomized to one of four groups: tryptophan + contingent vouchers, tryptophan + non-contingent vouchers, placebo + contingent vouchers, and placebo + non-contingent vouchers. The study included residential stabilization (4–9 days), where patients achieved initial cocaine abstinence; outpatient treatment evaluation (16 weeks), where patients received medication, vouchers, and urine testing thrice weekly; and disposition. Main outcomes were retention in treatment, urinalysis, self-reported drug use, and self-reported side effects. Tryptophan did not significantly prevent relapse to cocaine use or attenuate cocaine use after relapse. Contingent vouchers significantly increased the time to cocaine relapse and produced less cocaine use relative to non-contingent vouchers. Results demonstrate the sensitivity of this methodology for detecting decreases in cocaine use, as evidenced by significant changes of the contingent voucher conditions; this suggests that tryptophan's lack of efficacy was not due to model insensitivity for detecting significant differences in cocaine use. This study also showed that contingent vouchers were effective in the novel experimental model of relapse prevention.