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: It is essential to continue follow - up to quantify the long - term risks and benefits of tamoxifen therapy .
### Output Text:
It is essential to continue follow - up to quantify the long - term risks and benefits of <span class="chemical">tamoxifen</span> therapy . 