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: METHODS : We performed a systematic , retrospective study comparing active or former intravenous drug users receiving methadone and those not receiving methadone among all patients hospitalized over a 5 - year period in a tertiary care hospital .
### Output Text:
METHODS : We performed a systematic , retrospective study comparing active or former intravenous drug users receiving <span class="chemical">methadone</span> and those not receiving <span class="chemical">methadone</span> among all patients hospitalized over a 5 - year period in a tertiary care hospital . 