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: We describe eight patients in whom cocaine use was related to stroke and review 39 cases from the literature .
### Output Text:
We describe eight patients in whom <span class="chemical">cocaine</span> use was related to stroke and review 39 cases from the literature . 