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 frequency of sound - induced seizures after 12 . 5 or 25 mg / kg caffeine was reduced from 50 to 5 % by acetaminophen .
### Output Text:
The frequency of sound - induced seizures after 12 . 5 or 25 mg / kg <span class="chemical">caffeine</span> was reduced from 50 to 5 % by <span class="chemical">acetaminophen</span> . 