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: OBJECTIVE : The objective of this study was to compare the safety profiles of the 4 - mg nicotine lozenge and 4 - mg nicotine gum in smokers with selected label - restricted diseases .
### Output Text:
OBJECTIVE : The objective of this study was to compare the safety profiles of the 4 - mg <span class="chemical">nicotine</span> lozenge and 4 - mg <span class="chemical">nicotine</span> gum in smokers with selected label - restricted diseases . 