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: Headache was more severe in migraineurs than in the controls during and immediately after GTN infusion ( p = 0 . 037 ) as well as during the following 11 h ( p = 0 . 008 ) .
### Output Text:
Headache was more severe in migraineurs than in the controls during and immediately after <span class="chemical">GTN</span> infusion ( p = 0 . 037 ) as well as during the following 11 h ( p = 0 . 008 ) . 