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: In rare cases within the first few weeks of therapy , this drug can cause severe and reversible cholestatic jaundice .
### Output Text:
In rare cases within the first few weeks of therapy , this drug can cause severe and reversible cholestatic jaundice . 