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: We conclude that an H1 antagonist may be useful in preventing hypotension caused by iv cimetidine , since the vasodilating activity of cimetidine is mediated , in part , through the H1 receptor .
### Output Text:
We conclude that an H1 antagonist may be useful in preventing hypotension caused by iv <span class="chemical">cimetidine</span> , since the vasodilating activity of <span class="chemical">cimetidine</span> is mediated , in part , through the H1 receptor . 