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 incidence of adverse events in the 2 groups was similar during the first 2 weeks of product use ( evaluation population : 55 . 3 % lozenge , 54 . 7 % gum ) , as well as during the entire study ( safety population : 63 . 8 % and 58 . 6 % , respectively ) .
### Output Text:
The incidence of adverse events in the 2 groups was similar during the first 2 weeks of product use ( evaluation population : 55 . 3 % lozenge , 54 . 7 % gum ) , as well as during the entire study ( safety population : 63 . 8 % and 58 . 6 % , respectively ) . 