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: In addition to methadone dose , 15 demographic , biological , and pharmacological variables were considered as potential risk factors for QT prolongation .
### Output Text:
In addition to <span class="chemical">methadone</span> dose , 15 demographic , biological , and pharmacological variables were considered as potential risk factors for QT prolongation . 