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 there have been case reports of epidural morphine with these symptoms and signs , this has not been previously documented with IV or patient - controlled analgesia morphine .
### Output Text:
Although there have been case reports of epidural <span class="chemical">morphine</span> with these symptoms and signs , this has not been previously documented with IV or patient - controlled analgesia <span class="chemical">morphine</span> . 