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: Pretreatment with dexamethasone is not justified to prevent postoperative myalgia after succinylcholine .
### Output Text:
Pretreatment with <span class="chemical">dexamethasone</span> is not justified to prevent postoperative myalgia after <span class="chemical">succinylcholine</span> . 