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: When single - dose haloperidol was given again 25 days later , greater impairment with earlier onset was noted in several tests in both treatment groups , suggesting enhancement of this effect .
### Output Text:
When single - dose <span class="chemical">haloperidol</span> was given again 25 days later , greater impairment with earlier onset was noted in several tests in both treatment groups , suggesting enhancement of this effect . 