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: This potential side - effect , though very rare , represents one more reason to be very restrictive in the use of methylphenidate .
### Output Text:
This potential side - effect , though very rare , represents one more reason to be very restrictive in the use of <span class="chemical">methylphenidate</span> . 