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: For sumatriptan , tightness in the chest caused by an unknown mechanism has been reported in 3 - 5 % of users .
### Output Text:
For <span class="chemical">sumatriptan</span> , tightness in the chest caused by an unknown mechanism has been reported in 3 - 5 % of users . 