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: Although all of the currently available H2 - receptor antagonists have shown the propensity to cause delirium , only two previously reported cases have been associated with famotidine .
### Output Text:
Although all of the currently available H2 - receptor antagonists have shown the propensity to cause delirium , only two previously reported cases have been associated with <span class="chemical">famotidine</span> . 