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: Previously recognized side effects , particularly tremor and gastrointestinal problems , were more frequent in the mexiletine group than in the placebo group .
### Output Text:
Previously recognized side effects , particularly tremor and gastrointestinal problems , were more frequent in the <span class="chemical">mexiletine</span> group than in the placebo group . 