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: Caution should be exercised in using digoxin until large mortality trials are completed showing either benefit or harm .
### Output Text:
Caution should be exercised in using <span class="chemical">digoxin</span> until large mortality trials are completed showing either benefit or harm . 