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: While side effects were rare , those experienced after theophylline use did involve the cardiovascular and gastrointestinal systems .
### Output Text:
While side effects were rare , those experienced after <span class="chemical">theophylline</span> use did involve the cardiovascular and gastrointestinal systems . 