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: The co - primary endpoint of being pain free at 2 h was also in favor of rizatriptan .
### Output Text:
The co - primary endpoint of being pain free at 2 h was also in favor of <span class="chemical">rizatriptan</span> . 