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: We consider asterixis to be an easily overlooked sign of neurotoxicity , which may occur even at low or moderate dosage levels , if certain drugs as lithium or clozapine are used in combination with CBZ .
### Output Text:
We consider asterixis to be an easily overlooked sign of neurotoxicity , which may occur even at low or moderate dosage levels , if certain drugs as <span class="chemical">lithium</span> or <span class="chemical">clozapine</span> are used in combination with <span class="chemical">CBZ</span> . 