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: The underlying epileptogenecity of the patients or the synergistic effects of the accompanying drugs is required to trigger the de novo absence seizure .
### Output Text:
The underlying epileptogenecity of the patients or the synergistic effects of the accompanying drugs is required to trigger the de novo absence seizure . 