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: They had been taking metronidazole ( total dosage , 45 - 120 g ; duration , 30 days to 2 months ) to treat the infection in various organs .
### Output Text:
They had been taking <span class="chemical">metronidazole</span> ( total dosage , 45 - 120 g ; duration , 30 days to 2 months ) to treat the infection in various organs . 