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: Drowsiness was common on clonidine , but generally resolved by 6 to 8 weeks .
### Output Text:
Drowsiness was common on <span class="chemical">clonidine</span> , but generally resolved by 6 to 8 weeks . 