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: Prospective multicentre studies with a large population and a long follow - up should be performed in order to evaluate the incidence of this unusual side effect .
### Output Text:
Prospective multicentre studies with a large population and a long follow - up should be performed in order to evaluate the incidence of this unusual side effect . 