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: Physicians prescribing clonidine should monitor for bradycardia and advise patients about the high likelihood of initial drowsiness .
### Output Text:
Physicians prescribing <span class="chemical">clonidine</span> should monitor for bradycardia and advise patients about the high likelihood of initial drowsiness . 