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 novel model of OCD demonstrates that drug exposure during a sensitive period can program disease - like systems permanently , which could have implications for current and future therapeutic strategies for this and other psychiatric disorders .
### Output Text:
This novel model of OCD demonstrates that drug exposure during a sensitive period can program disease - like systems permanently , which could have implications for current and future therapeutic strategies for this and other psychiatric disorders . 