A large body of research has shown that methadone effectively reduces opioid use and opioid-related harms. As an example, a systematic review of 11 studies found that compared to no medication for opioid use disorder, treatment with methadone significantly reduced heroin use as measured both by self-report and urine/hair analysis.

Perhaps the most important goal of opioid use disorder treatment is to reduce the risk of opioid overdose and death. Compared to taking no medications for opioid use disorder, taking either buprenorphine or methadone is associated with lower odds of opioid-related overdose. In a large national observational study, opioid-related overdoses over three months were reduced by 76% among people taking either buprenorphine or methadone compared to no medication. Studies have also evaluated the outcome of opioid-related deaths. In a study of Massachusetts data, methadone treatment was associated with a 59% reduction in opioid-related mortality over 12 months compared to no medications. A larger systematic review and meta-analysis combining data across 51 randomized trials and observational studies found the same 53% reduction in opioid-related mortality.  

Another major goal in opioid use disorder treatment is to reduce opioid-related healthcare encounters; this is relevant to an individual’s personal health but is also an important indicator of an individual’s ability to spend time on the activities that are most important to them. In a large national observational study, opioid-related acute care use was defined as an emergency department visit or hospitalization for an opioid-related reason. People taking either buprenorphine or methadone had a 32% reduction in opioid-related acute care use. 

Treatment retention, or the duration of time individuals stay in treatment, is typically measured by attendance at outpatient appointments over a defined period. In a systematic review and meta-analysis of 13 randomized trials and observational studies, the average retention rate of individuals taking methadone varied between 30% and 80%, which was similar to the average retention rate for buprenorphine, ranging from 20% to 80%. Another systematic review and meta-analysis found that over longer periods of time, such as 6 months, treatment retention for people taking methadone was slightly better than for buprenorphine. 

Several studies have shown that persons with opioid use disorder have substantial physical and psychological impairment that affects their quality of life, which is greater than the impairment experienced by patients with chronic obstructive pulmonary disease or coronary artery disease. Encouragingly, studies have found that quality of life improves substantially in patients in methadone maintenance treatment.