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: Pain responses in methadone - maintained opioid abusers .
### Output Text:
Pain responses in <span class="chemical">methadone</span> - maintained opioid abusers . 