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: Follow - up MRIs were performed on 5 patients from third to 14th days after discontinuation of metronidazole administration .
### Output Text:
Follow - up MRIs were performed on 5 patients from third to 14th days after discontinuation of <span class="chemical">metronidazole</span> administration . 