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: CONCLUSIONS : Our results suggest that the antagonistic effect of nicotine on caffeine - induced anxiety is specific to caffeine , instead of a non - specific anxiolytic effect .
### Output Text:
CONCLUSIONS : Our results suggest that the antagonistic effect of <span class="chemical">nicotine</span> on <span class="chemical">caffeine</span> - induced anxiety is specific to <span class="chemical">caffeine</span> , instead of a non - specific anxiolytic effect . 