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: On repeat challenge with phenobarbital , the dyskinesia recurred .
### Output Text:
On repeat challenge with <span class="chemical">phenobarbital</span> , the dyskinesia recurred . 