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: Multiple cytotoxic drug administration is the generally accepted treatment of patients with a high - risk stage of choriocarcinoma .
### Output Text:
Multiple cytotoxic drug administration is the generally accepted treatment of patients with a high - risk stage of choriocarcinoma . 