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: CONCLUSION : The 4 - mg nicotine lozenge and 4 - mg nicotine gum had comparable safety profiles in these patients with label - restricted medical conditions .
### Output Text:
CONCLUSION : The 4 - mg <span class="chemical">nicotine</span> lozenge and 4 - mg <span class="chemical">nicotine</span> gum had comparable safety profiles in these patients with label - restricted medical conditions . 