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: Oral rehabilitation of patients using methamphetamine can be challenging .
### Output Text:
Oral rehabilitation of patients using <span class="chemical">methamphetamine</span> can be challenging . 