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: However , any improvement produced by drug treatment was largely a result of a beneficial effect exerted by its vehicle ( polyethylene glycol and NaOH ) .
### Output Text:
However , any improvement produced by drug treatment was largely a result of a beneficial effect exerted by its vehicle ( <span class="chemical">polyethylene glycol</span> and <span class="chemical">NaOH</span> ) . 