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: Stratification of patients by sex , age , extent of concurrent smoking , extent of product use , and severity of adverse events revealed no clinically significant differences between the lozenge and gum .
### Output Text:
Stratification of patients by sex , age , extent of concurrent smoking , extent of product use , and severity of adverse events revealed no clinically significant differences between the lozenge and gum . 