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: Cocaine ( 40 mg / kg ) was injected subcutaneously for an acute experiment and subsequent 20 mg / kg doses twice daily for 3 days in a chronic study .
### Output Text:
<span class="chemical">Cocaine</span> ( 40 mg / kg ) was injected subcutaneously for an acute experiment and subsequent 20 mg / kg doses twice daily for 3 days in a chronic study . 