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: Long QT syndrome can occur with low doses of methadone .
### Output Text:
Long QT syndrome can occur with low doses of <span class="chemical">methadone</span> . 