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: Although some studies have suggested fewer venous complications are associated with midazolam than with diazepam for endoscopic procedures , this variable has not been well documented .
### Output Text:
Although some studies have suggested fewer venous complications are associated with <span class="chemical">midazolam</span> than with <span class="chemical">diazepam</span> for endoscopic procedures , this variable has not been well documented . 