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: Moreover , the coadministration of these frequently used drugs is expected to be especially harmful in this subgroup of patients .
### Output Text:
Moreover , the coadministration of these frequently used drugs is expected to be especially harmful in this subgroup of patients . 