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: The most common drug - related adverse events were diarrhea , flatulence , and nausea ; most adverse events were mild to moderate in severity .
### Output Text:
The most common drug - related adverse events were diarrhea , flatulence , and nausea ; most adverse events were mild to moderate in severity . 