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: In the current study , dyskinetic patients performed better in swallowing function , which could be explained on the basis of a greater levodopa dose .
### Output Text:
In the current study , dyskinetic patients performed better in swallowing function , which could be explained on the basis of a greater <span class="chemical">levodopa</span> dose . 