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: Extrapyramidal side effects with risperidone and haloperidol at comparable D2 receptor occupancy levels .
### Output Text:
Extrapyramidal side effects with <span class="chemical">risperidone</span> and <span class="chemical">haloperidol</span> at comparable D2 receptor occupancy levels . 