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: Prolongation of the QT interval in the ECG of patients with torsade de pointes ( TdP ) has been reported in methadone users .
### Output Text:
Prolongation of the QT interval in the ECG of patients with torsade de pointes ( TdP ) has been reported in <span class="chemical">methadone</span> users . 