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: We conclude that lidocaine reduces the incidence and severity of propofol injection pain in ambulatory patients whereas thiopentone only reduces its severity .
### Output Text:
We conclude that <span class="chemical">lidocaine</span> reduces the incidence and severity of <span class="chemical">propofol</span> injection pain in ambulatory patients whereas <span class="chemical">thiopentone</span> only reduces its severity . 