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 antiepileptic drugs , phenobarbitone and carbamazepine are well known to cause cognitive impairment on chronic use .
### Output Text:
The antiepileptic drugs , <span class="chemical">phenobarbitone</span> and <span class="chemical">carbamazepine</span> are well known to cause cognitive impairment on chronic use . 