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: As phenytoin is a commonly used drug , clinicians need to be aware of this interaction .
### Output Text:
As <span class="chemical">phenytoin</span> is a commonly used drug , clinicians need to be aware of this interaction . 