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: Multivariate regression analysis allowed attribution of 31 . 8 % of QTc variability to methadone dose , cytochrome P - 450 3A4 drug - drug interactions , hypokalemia , and altered liver function .
### Output Text:
Multivariate regression analysis allowed attribution of 31 . 8 % of QTc variability to <span class="chemical">methadone</span> dose , cytochrome P - 450 3A4 drug - drug interactions , hypokalemia , and altered liver function . 