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: METHOD : Six patients with either obsessive compulsive disorder , trichotillomania , anxiety , or affective disorders who suffered sexual side effects after treatment with serotonin reuptake blockers were given yohimbine on a p . r . n . basis in an open clinical trial .
### Output Text:
METHOD : Six patients with either obsessive compulsive disorder , trichotillomania , anxiety , or affective disorders who suffered sexual side effects after treatment with <span class="chemical">serotonin</span> reuptake blockers were given <span class="chemical">yohimbine</span> on a p . r . n . basis in an open clinical trial . 