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 nicotine lozenge and nicotine gum were equally well tolerated , despite increased nicotine exposure from the lozenge .
### Output Text:
The <span class="chemical">nicotine</span> lozenge and <span class="chemical">nicotine</span> gum were equally well tolerated , despite increased <span class="chemical">nicotine</span> exposure from the lozenge . 