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: CASE SUMMARY : A 40 - year - old woman with major depression took an overdose of venlafaxine in an apparent suicide attempt .
### Output Text:
CASE SUMMARY : A 40 - year - old woman with major depression took an overdose of <span class="chemical">venlafaxine</span> in an apparent suicide attempt . 