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 an open , cross - over , controlled design , patients were randomized to receive either DCF per os or GTN patches the first days of menses , when menstrual cramps became unendurable .
### Output Text:
In an open , cross - over , controlled design , patients were randomized to receive either <span class="chemical">DCF</span> per os or <span class="chemical">GTN</span> patches the first days of menses , when menstrual cramps became unendurable . 