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: Effect of switching carbamazepine to oxcarbazepine on the plasma levels of neuroleptics .
### Output Text:
Effect of switching <span class="chemical">carbamazepine</span> to <span class="chemical">oxcarbazepine</span> on the plasma levels of neuroleptics . 