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: OBJECTIVE : To examine the effect of raloxifene on major adverse events that occur with postmenopausal estrogen therapy or tamoxifen .
### Output Text:
OBJECTIVE : To examine the effect of <span class="chemical">raloxifene</span> on major adverse events that occur with postmenopausal <span class="chemical">estrogen</span> therapy or <span class="chemical">tamoxifen</span> . 