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: Patients under methadone maintenance therapy should be warned regarding these serious adverse events .
### Output Text:
Patients under <span class="chemical">methadone</span> maintenance therapy should be warned regarding these serious adverse events . 