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: These changes were maximal in the hour immediately after medications and slowly returned toward base - line levels thereafter .
### Output Text:
These changes were maximal in the hour immediately after medications and slowly returned toward base - line levels thereafter . 