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: Eight patients stopped using GTN because headache - - attributed to its use - - became intolerable .
### Output Text:
Eight patients stopped using <span class="chemical">GTN</span> because headache - - attributed to its use - - became intolerable . 