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: He had used heroin since age 20 , and had used 150 mg methadone daily for 6 months .
### Output Text:
He had used <span class="chemical">heroin</span> since age 20 , and had used 150 mg <span class="chemical">methadone</span> daily for 6 months . 