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: Diazepam , unless contraindicated or risky , remains the only necessary complementary drug to ketamine as it buffers its cardiovascular response and decreases the duration and intensity of operative and postoperative hallucinations .
### Output Text:
<span class="chemical">Diazepam</span> , unless contraindicated or risky , remains the only necessary complementary drug to <span class="chemical">ketamine</span> as it buffers its cardiovascular response and decreases the duration and intensity of operative and postoperative hallucinations . 