"However, one problem markedly reduces naltrexone’s efficacy and has limited its use for treating heroin and other forms of opioid dependence worldwide: patients often do not like it and do not take it on a daily basis. The dropout rate with oral naltrexone has been better in the limited number of patients in whom there is substantial external motivation to remain abstinent, such as physicians who are in monitoring programs and could lose their license if they relapse, those involved in the criminal justice system who could go to prison if they relapse, and those facing loss of employment [1•, 2–4].
"A few US studies have shown positive effects with psychosocial or behavioral therapies. In two, contingency management combined with naltrexone was helpful [5, 6]. In another, naltrexone combined with individual  and group  psychotherapy yielded positive effects. A third tested a behavioral therapy that used rewards for negative urine tests ; however, it had a relatively limited effect and was identified by Nunes et al.  as one of several examples indicating that there appears to be a ceiling effect on the degree to which behavioral interventions can be used to improve naltrexone treatment outcomes."
Krupitsky, Evgeny, Zvartau, Edwin, and Woody, George, "Use of Naltrexone to Treat Opioid Addiction in a Country in Which Methadone and Buprenorphine Are Not Available," Curr Psychiatry Rep. 2010 October; 12(5): 448–453. doi:10.1007/s11920-010-0135-5.