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: Absences were aggravated in both cases where vigabatrin was added on to concurrent treatment .
### Output Text:
Absences were aggravated in both cases where <span class="chemical">vigabatrin</span> was added on to concurrent treatment . 