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: Pain perception and nerve conduction velocity in the tail were significantly impaired , particularly after the high - dose treatment .
### Output Text:
Pain perception and nerve conduction velocity in the tail were significantly impaired , particularly after the high - dose treatment . 