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: However , the long - term use of this dopamine precursor is complicated by highly disabling fluctuations and dyskinesias .
### Output Text:
However , the long - term use of this <span class="chemical">dopamine</span> precursor is complicated by highly disabling fluctuations and dyskinesias . 