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: Carbamazepine is a safe drug , but physicians and patients should be aware of the exceedingly rare but potentially fatal side effects , better prevented by clinical than by laboratory monitoring .
### Output Text:
<span class="chemical">Carbamazepine</span> is a safe drug , but physicians and patients should be aware of the exceedingly rare but potentially fatal side effects , better prevented by clinical than by laboratory monitoring . 