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: The combination inhibited intestinal transit similar to that produced by morphine regardless of the time of treatment , suggesting that metamizol did not potentiate morphine - induced constipation .
### Output Text:
The combination inhibited intestinal transit similar to that produced by <span class="chemical">morphine</span> regardless of the time of treatment , suggesting that <span class="chemical">metamizol</span> did not potentiate <span class="chemical">morphine</span> - induced constipation . 