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 findings suggest that amphetamine has an irreversible effect on the post - lesion DA pool contributing to contralateral rotation .
### Output Text:
These findings suggest that <span class="chemical">amphetamine</span> has an irreversible effect on the post - lesion DA pool contributing to contralateral rotation . 