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: Until physicians began reporting an unusual acute flank pain syndrome to the spontaneous reporting system , 700 , 000 persons used the drug in the United States .
### Output Text:
Until physicians began reporting an unusual acute flank pain syndrome to the spontaneous reporting system , 700 , 000 persons used the drug in the United States . 