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: This study evaluated yohimbine as a treatment for the sexual side effects caused by serotonin reuptake blockers .
### Output Text:
This study evaluated <span class="chemical">yohimbine</span> as a treatment for the sexual side effects caused by <span class="chemical">serotonin</span> reuptake blockers . 