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: A total of 167 patients receiving methadone fulfilled the inclusion criteria and were compared with a control group of 80 injection drug users not receiving methadone .
### Output Text:
A total of 167 patients receiving <span class="chemical">methadone</span> fulfilled the inclusion criteria and were compared with a control group of 80 injection drug users not receiving <span class="chemical">methadone</span> . 