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: Somnolence and constipation were prominent toxicities and most patients could not tolerate the 1200 mg / day dose level .
### Output Text:
Somnolence and constipation were prominent toxicities and most patients could not tolerate the 1200 mg / day dose level . 