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: Following the initial period of therapy , emerging difficulties require a reassessment of therapeutic approaches , such as dosage adjustment or introduction of a dopamine agonist .
### Output Text:
Following the initial period of therapy , emerging difficulties require a reassessment of therapeutic approaches , such as dosage adjustment or introduction of a <span class="chemical">dopamine</span> agonist . 