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: Regimens commonly used often require inpatient administration and can be difficult to deliver due to toxicity .
### Output Text:
Regimens commonly used often require inpatient administration and can be difficult to deliver due to toxicity . 