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: Renal structure and concentrating ability were examined after a recovery period of up to 18 weeks , when no analgesics were given , to investigate whether the analgesic - induced changes were reversible .
### Output Text:
Renal structure and concentrating ability were examined after a recovery period of up to 18 weeks , when no analgesics were given , to investigate whether the analgesic - induced changes were reversible . 