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 efficacy of prochlorperazine in the treatment of headache and nausea likewise did not appear to be affected by the rate of administration , although no formal statistical comparisons were made .
### Output Text:
The efficacy of <span class="chemical">prochlorperazine</span> in the treatment of headache and nausea likewise did not appear to be affected by the rate of administration , although no formal statistical comparisons were made . 