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: As heroin addicts sometimes faint while using illicit drugs , doctors might attribute too many episodes of syncope to illicit drug use and thereby underestimate the incidence of TdP in this special population , and the high mortality in this population may , in part , be caused by the proarrhythmic effect of methadone .
### Output Text:
As <span class="chemical">heroin</span> addicts sometimes faint while using illicit drugs , doctors might attribute too many episodes of syncope to illicit drug use and thereby underestimate the incidence of TdP in this special population , and the high mortality in this population may , in part , be caused by the proarrhythmic effect of <span class="chemical">methadone</span> . 