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: These findings indicate that GTN has a reduced efficacy and tolerability by comparison with DCF in the treatment of primary dysmenorrhea .
### Output Text:
These findings indicate that <span class="chemical">GTN</span> has a reduced efficacy and tolerability by comparison with <span class="chemical">DCF</span> in the treatment of primary dysmenorrhea . 