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 withdrawal and the gradual reintroduction of quetiapine 2 weeks later , which led to another severe agitation , enabled us to attribute the agitation specifically to quetiapine .
### Output Text:
The withdrawal and the gradual reintroduction of <span class="chemical">quetiapine</span> 2 weeks later , which led to another severe agitation , enabled us to attribute the agitation specifically to <span class="chemical">quetiapine</span> . 