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: Patients were randomized in a 1 : 1 ratio to receive the 4 - mg nicotine lozenge or 4 - mg nicotine gum .
### Output Text:
Patients were randomized in a 1 : 1 ratio to receive the 4 - mg <span class="chemical">nicotine</span> lozenge or 4 - mg <span class="chemical">nicotine</span> gum . 