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: The half - life ( t1 / 2 ) of cocaine is relatively short , but some of the consequences of its use , such as seizures and strokes , can occur hours after exposure .
### Output Text:
The half - life ( t1 / 2 ) of <span class="chemical">cocaine</span> is relatively short , but some of the consequences of its use , such as seizures and strokes , can occur hours after exposure . 