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: At admission simvastatin and all antiviral drugs were discontinued because toxicity due to a drug - drug interaction was suspected .
### Output Text:
At admission <span class="chemical">simvastatin</span> and all antiviral drugs were discontinued because toxicity due to a drug - drug interaction was suspected . 