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: Among the subjects treated with methadone , 28 % men and 32 % women had prolonged QTc interval .
### Output Text:
Among the subjects treated with <span class="chemical">methadone</span> , 28 % men and 32 % women had prolonged QTc interval . 