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: Oxitropium proves to be a valuable alternative to theophylline in nocturnal asthma , since it is equally potent , safer and does not require the titration of dosage .
### Output Text:
<span class="chemical">Oxitropium</span> proves to be a valuable alternative to <span class="chemical">theophylline</span> in nocturnal asthma , since it is equally potent , safer and does not require the titration of dosage . 