In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: This study was designed to describe pain tolerance and analgesic response in a sample of opioid addicts stabilized in methadone - maintenance ( MM ) treatment ( n = 60 ) in comparison to matched nondependent control subjects ( n = 60 ) .
### Output Text:
This study was designed to describe pain tolerance and analgesic response in a sample of opioid addicts stabilized in <span class="chemical">methadone</span> - maintenance ( MM ) treatment ( n = 60 ) in comparison to matched nondependent control subjects ( n = 60 ) . 