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: In patients with impaired renal and liver function , and those who need long - term pain control , pethidine may cause excitatory central nervous system ( CNS ) effects through its neurotoxic metabolite , norpethidine , resulting in irritability and seizure attack .
### Output Text:
In patients with impaired renal and liver function , and those who need long - term pain control , <span class="chemical">pethidine</span> may cause excitatory central nervous system ( CNS ) effects through its neurotoxic metabolite , <span class="chemical">norpethidine</span> , resulting in irritability and seizure attack . 