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: This method allowed frequent self - dosing of pethidine at short time intervals and rapid accumulation of pethidine and norpethidine .
### Output Text:
This method allowed frequent self - dosing of <span class="chemical">pethidine</span> at short time intervals and rapid accumulation of <span class="chemical">pethidine</span> and <span class="chemical">norpethidine</span> . 