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: CONCLUSIONS : The venlafaxine overdose in our patient resulted in a single episode of generalized seizure but elicited no further sequelae .
### Output Text:
CONCLUSIONS : The <span class="chemical">venlafaxine</span> overdose in our patient resulted in a single episode of generalized seizure but elicited no further sequelae . 