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: High doses of the vitamin K antagonist Warfarin are also known to cause calcification of the artery media , but at treatment times of 2 weeks or longer yet not at 1 week .
### Output Text:
High doses of the <span class="chemical">vitamin K</span> antagonist <span class="chemical">Warfarin</span> are also known to cause calcification of the artery media , but at treatment times of 2 weeks or longer yet not at 1 week . 