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: 15 cases of cimetidine - associated mental confusion have been reported .
### Output Text:
15 cases of <span class="chemical">cimetidine</span> - associated mental confusion have been reported . 