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 possible interaction of tizanidine and other antihypertensive agents should be kept in mind when prescribing therapy to treat either hypertension or spasticity in such patients .
### Output Text:
The possible interaction of <span class="chemical">tizanidine</span> and other antihypertensive agents should be kept in mind when prescribing therapy to treat either hypertension or spasticity in such patients . 