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: METHODS : Forty patients with Parkinson ' s disease underwent serial , detailed assessments both after drug withdrawal ( " off " period ) and while taking their optimal medical regimens ( " on " period ) .
### Output Text:
METHODS : Forty patients with Parkinson ' s disease underwent serial , detailed assessments both after drug withdrawal ( " off " period ) and while taking their optimal medical regimens ( " on " period ) . 