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: Large differences , regarded as statistically significant , between the mexiletine and placebo groups were noted in that end point at months 1 and 4 , but only trends were observed at month 12 .
### Output Text:
Large differences , regarded as statistically significant , between the <span class="chemical">mexiletine</span> and placebo groups were noted in that end point at months 1 and 4 , but only trends were observed at month 12 . 