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: Moderate or severe adverse events were more common in subjects on clonidine ( 79 . 4 % versus 49 . 2 % ; p = . 0006 ) but not associated with higher rates of early study withdrawal .
### Output Text:
Moderate or severe adverse events were more common in subjects on <span class="chemical">clonidine</span> ( 79 . 4 % versus 49 . 2 % ; p = . 0006 ) but not associated with higher rates of early study withdrawal . 