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: In the controls , the GTN - induced headache gradually disappeared , whereas in migraineurs peak headache intensity occurred at a mean time of 240 min post - infusion .
### Output Text:
In the controls , the <span class="chemical">GTN</span> - induced headache gradually disappeared , whereas in migraineurs peak headache intensity occurred at a mean time of 240 min post - infusion . 