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: A large population - based follow - up study of trimethoprim - sulfamethoxazole , trimethoprim , and cephalexin for uncommon serious drug toxicity .
### Output Text:
A large population - based follow - up study of <span class="chemical">trimethoprim - sulfamethoxazole</span> , <span class="chemical">trimethoprim</span> , and <span class="chemical">cephalexin</span> for uncommon serious drug toxicity . 