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 symptoms may be due to ' sensitisation ' following repeated levodopa treatment or a direct effect of dopamine on the disease state .
### Output Text:
These symptoms may be due to ' sensitisation ' following repeated <span class="chemical">levodopa</span> treatment or a direct effect of <span class="chemical">dopamine</span> on the disease state . 