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: Safety profile of a nicotine lozenge compared with that of nicotine gum in adult smokers with underlying medical conditions : a 12 - week , randomized , open - label study .
### Output Text:
Safety profile of a <span class="chemical">nicotine</span> lozenge compared with that of <span class="chemical">nicotine</span> gum in adult smokers with underlying medical conditions : a 12 - week , randomized , open - label study . 