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: He had been well until the morning of presentation when he took 1 / 2 tab of venlafaxine .
### Output Text:
He had been well until the morning of presentation when he took 1 / 2 tab of <span class="chemical">venlafaxine</span> . 