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: RESULTS : Among 167 methadone maintenance patients , the prevalence of QTc prolongation to 0 . 50 second ( ( 1 / 2 ) ) or longer was 16 . 2 % compared with 0 % in 80 control subjects .
### Output Text:
RESULTS : Among 167 <span class="chemical">methadone</span> maintenance patients , the prevalence of QTc prolongation to 0 . 50 second ( ( 1 / 2 ) ) or longer was 16 . 2 % compared with 0 % in 80 control subjects . 