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 aim of this study was to evaluate the relationship between phenytoin medication and cerebellar atrophy in patients who had experienced clinical intoxication .
### Output Text:
The aim of this study was to evaluate the relationship between <span class="chemical">phenytoin</span> medication and cerebellar atrophy in patients who had experienced clinical intoxication . 