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: While the mutual potentiation , antagonism and tolerance suggest common mechanisms for the induced catalepsy , differences in latency , rigidity and behavior , asymmetry of cross - tolerance and a widely - different ID50 for naloxone would argue against an action at a single opioid site .
### Output Text:
While the mutual potentiation , antagonism and tolerance suggest common mechanisms for the induced catalepsy , differences in latency , rigidity and behavior , asymmetry of cross - tolerance and a widely - different ID50 for <span class="chemical">naloxone</span> would argue against an action at a single opioid site . 