The Opoid Substitution Therapy (OST) is a medicine (Beuprenorphin-BPN) used by IDUs under the tongue with the consult of expert medical officer. The OST has been recognized worldwide as an effective treatment for opioid dependence and harm reduction strategy. OST involves substitution of illicit, unsafe opioid drugs which may be taken through injecting route with a legal medicine which has similar properties as that of the injection opioid but taken with a safer route. The medicine is prescribed by a doctor and administered under the supervision of a trained health-care staff in such doses that the opioid dependent IDUs would not have craving or withdrawals. As the medicine takes care of the drug hunger, IDUs stop or reduce injecting drugs, leading to decreased risk of transmission of HIV and other blood borne viruses. However, the benefit of OST goes beyond prevention of HIV and other blood borne viruses. OST has been successful in reducing other harms related to drug, especially when combined with psychosocial interventions. The client on OST stops indulging in petty crimes, becomes stabilized psychologically, becomes able to think coherently, becomes able to take care of his family and also becomes gainfully employed.