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: Seventy - one percent of the patients reported that the behavioral effects of caffeine were similar to those experienced during panic attacks .
### Output Text:
Seventy - one percent of the patients reported that the behavioral effects of <span class="chemical">caffeine</span> were similar to those experienced during panic attacks . 