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 maximum effects of each treatment occurred at the time of maximum phenytoin sodium levels .
### Output Text:
The maximum effects of each treatment occurred at the time of maximum <span class="chemical">phenytoin sodium</span> levels . 