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: Both patients and controls developed significantly stronger immediate headache on the GTN day than on the placebo day and the headache was significantly more pronounced in patients than in controls .
### Output Text:
Both patients and controls developed significantly stronger immediate headache on the <span class="chemical">GTN</span> day than on the placebo day and the headache was significantly more pronounced in patients than in controls . 