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: This complication reappeared on day 25 during the second dose of 5 - fluorouracil and folinic acid , which were then the only drugs given .
### Output Text:
This complication reappeared on day 25 during the second dose of <span class="chemical">5 - fluorouracil</span> and <span class="chemical">folinic acid</span> , which were then the only drugs given . 