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: The association between opioid dose and QT , and methadone dose and reporting of syncope was assessed using multivariate linear regression and logistic regression , respectively .
### Output Text:
The association between opioid dose and QT , and <span class="chemical">methadone</span> dose and reporting of syncope was assessed using multivariate linear regression and logistic regression , respectively . 