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: A long - term evaluation of the risk - benefit ratio of tamoxifen as a preventive treatment for breast cancer is clearly warranted .
### Output Text:
A long - term evaluation of the risk - benefit ratio of <span class="chemical">tamoxifen</span> as a preventive treatment for breast cancer is clearly warranted . 