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: No subject reported side effects of oxitropium , as compared to three subjects reporting nausea , vomiting and tremors after theophylline .
### Output Text:
No subject reported side effects of <span class="chemical">oxitropium</span> , as compared to three subjects reporting nausea , vomiting and tremors after <span class="chemical">theophylline</span> . 