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 effect of naloxone could be antagonized with morphine ( 1 , 3 , and 10 mg / kg ) , demonstrating the opioid specificity of the naloxone effect .
### Output Text:
The effect of <span class="chemical">naloxone</span> could be antagonized with <span class="chemical">morphine</span> ( 1 , 3 , and 10 mg / kg ) , demonstrating the opioid specificity of the <span class="chemical">naloxone</span> effect . 