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: A lower relative risk would be expected for acetaminophen if the risk of both drugs in combination with other analgesics was higher than the risk of either agent alone .
### Output Text:
A lower relative risk would be expected for <span class="chemical">acetaminophen</span> if the risk of both drugs in combination with other analgesics was higher than the risk of either agent alone . 