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: In the current study , we investigated the synergy between these 2 treatments and found that concurrent Warfarin administration dramatically increased the extent of calcification in the media of vitamin D - treated rats at 3 and 4 days .
### Output Text:
In the current study , we investigated the synergy between these 2 treatments and found that concurrent <span class="chemical">Warfarin</span> administration dramatically increased the extent of calcification in the media of <span class="chemical">vitamin D</span> - treated rats at 3 and 4 days . 