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: This uncoupling effect on oxidative phosphorylation was not detectable after short - term exposure to cocaine , suggesting that these mitochondrial abnormalities were a late rather than a primary event in the pathological response to cocaine .
### Output Text:
This uncoupling effect on oxidative phosphorylation was not detectable after short - term exposure to <span class="chemical">cocaine</span> , suggesting that these mitochondrial abnormalities were a late rather than a primary event in the pathological response to <span class="chemical">cocaine</span> . 