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: The high ratio is believed to impart the low frequency of extrapyramidal symptoms with risperidone at low dosages .
### Output Text:
The high ratio is believed to impart the low frequency of extrapyramidal symptoms with <span class="chemical">risperidone</span> at low dosages . 