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 data demonstrate that administration of MP before and after TDI reduces the frequency and severity of the arthralgia - myalgia syndrome .
### Output Text:
These data demonstrate that administration of <span class="chemical">MP</span> before and after TDI reduces the frequency and severity of the arthralgia - myalgia syndrome . 