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: CONCLUSION : Serious hepatotoxicity associated with sulfasalazine appears to be under - appreciated and intensive monitoring and vigilance in the first 6 weeks of treatment is especially important .
### Output Text:
CONCLUSION : Serious hepatotoxicity associated with <span class="chemical">sulfasalazine</span> appears to be under - appreciated and intensive monitoring and vigilance in the first 6 weeks of treatment is especially important . 