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 , rapid pain relief sometimes brings about unfavourable side effects that largely limit its clinical utility .
### Output Text:
However , rapid pain relief sometimes brings about unfavourable side effects that largely limit its clinical utility . 