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: These differences were observed even though the serum mexiletine levels obtained in this study were generally lower than those observed in studies that have used the regular form of the drug .
### Output Text:
These differences were observed even though the serum <span class="chemical">mexiletine</span> levels obtained in this study were generally lower than those observed in studies that have used the regular form of the drug . 