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: Because of ribavirin - related hemolytic anemia , dose reduction is a common event in this therapy .
### Output Text:
Because of <span class="chemical">ribavirin</span> - related hemolytic anemia , dose reduction is a common event in this therapy . 