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: This may be because cimetidine acts as a histamine agonist .
### Output Text:
This may be because <span class="chemical">cimetidine</span> acts as a <span class="chemical">histamine</span> agonist . 