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: Digoxin has a narrow therapeutic : toxic ratio and concentrations are affected by a number of drugs .
### Output Text:
<span class="chemical">Digoxin</span> has a narrow therapeutic : toxic ratio and concentrations are affected by a number of drugs . 