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: RESULTS : Hyperprolactinemia was present in 49 % of 83 boys ( n = 41 ) treated with risperidone for a mean of 2 . 9 years .
### Output Text:
RESULTS : Hyperprolactinemia was present in 49 % of 83 boys ( n = 41 ) treated with <span class="chemical">risperidone</span> for a mean of 2 . 9 years . 