According to the World Drug Report, approximately 200 million people, about 5% of the World’s population aged between 15 and 64 years have either been forced or voluntarily sought the help of rehabilitation centers for behavioral modifications [1]. These rehabilitation centers over the years have admitted drug addicts into their rehabilitation programs meant to bring about behavior change among the rehabilitees. However, little is known about the effectiveness of drug rehabilitation programs on behavior modification of drug addicts in rehabilitation centers in Kisii County and its environs. Subsequently, no one has established whether the existing rehabilitation programs put in place to help the drug addicts are functional. The study employed the cross sectional design as a method of data collection. The study population was the staff and the rehabilitees of purposively sampled ten rehabilitation centers in Kisii County and its environs. A pre-test was carried out in three randomly selected rehabilitation centers which were not to be part of the final study group. Questionnaires were used as tools for collecting data, one for the staff and questionnaire-guided interviews for the rehabilitees. Data was coded and entered in to the Statistical Package for Social Scientists (SPSS Ver. 10) software and descriptive statistics such as frequencies and percentages calculated. The study employed the matrix model to achieve its objectives. The study sought to prove the Cognitive Theory which has profound implications for the rehabilitees in rehabilitation centers in Kisii County and its environs. The study mainly found that; Drug users ’evaluation in terms of assessment emerged the key programs offered by rehabilitation centers in Kisii County and its environs; staff members in rehabilitation centers in Kisii County were sufficiently trained, with a good number of them holding degrees, diplomas and certificates from reputable institutions. They also possess proficiency in drug management and rehabilitation tasks. Rehabilitation centers in Kisii County and its environs carried personal continuing / aftercare services to their clients using psychological interventions, and individual personal empowerment of the rehabilitee. Financial impediments like lack of sufficient staff, medication and facilities as well as inconsistent follow ups are major challenges facing rehabilitation centers in Kisii County. These challenges can only be curbed by sufficient funding of these institutions to enable employment of adequate staff and acquisition of better infrastructure to enable enhanced individual attention to rehabilitees. The study recommended that; rehabilitation centers management should foster establishment and implementation of a comprehensive drug rehabilitation schedule that caters to the individual needs of a rehabilitee and at the same time manage rehab time to avoid boredom among the patients. Rehabilitation centers staff should gain insights on various needs of the rehabilitees to avoid unnecessary strife in the rehabs. They should also learn to handle the rehabilitees with professionalism to enable a successful rehabilitation process. Rehabilitees in the rehabilitation program should also learn a number of personal management tips to help them adequately plan for and make the most out of personal management tips for the period they are in the rehab center. The ministry of special programs should look into various problems encountered by staff and clients in rehabilitation centers. The ministry should then formulate policies on the funding and staffing of these centers to facilitate a sound drug rehabilitation process.