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: For almost all cognitive measures , there were no medication by age - interaction effects , which indicates that the 2 age groups exhibited similar responses to the medication challenge .
### Output Text:
For almost all cognitive measures , there were no medication by age - interaction effects , which indicates that the 2 age groups exhibited similar responses to the medication challenge . 