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: Patients who were using a higher dose of levodopa had a greater OPSE and a trend toward a smaller oral transit time ( Pearson ' s correlation , P = 0 . 01 and 0 . 08 , respectively ) .
### Output Text:
Patients who were using a higher dose of <span class="chemical">levodopa</span> had a greater OPSE and a trend toward a smaller oral transit time ( Pearson ' s correlation , P = 0 . 01 and 0 . 08 , respectively ) . 