Background: Studies have shown that methadone maintenance treatment (MMT) is effective in improving the client’s quality of life and physical health. This study aimed to describe the nature and structure of drug dependents' experiences and the physical effects of MMT Methods: The present study is a qualitative and a phenomenology study on 32 clients referred to methadone clinics in the city of Kerman in 2008. Colaizzi’s method was used for data analysis and to evaluate the data, validity and reliability criteria were used. Findings: Encoded concepts were categorized in general groups of effectiveness on general health, sleep, appetite and weight, sexual desire, appearance and other effects. These six categories showed the main structure of experience and physical effects of MMT. Conclusion: The clients' viewpoints towards this treatment had a role in their experience expression and feelings, but MMT had an overall positive physical effect on the clients Keywords: Methadone maintenance treatment, Physical effects.