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: Pooled data from trials comparing antithrombotic treatment with placebo have shown that warfarin reduces the risk of stroke by 62 % , and that aspirin alone reduces the risk by 22 % .
### Output Text:
Pooled data from trials comparing antithrombotic treatment with placebo have shown that <span class="chemical">warfarin</span> reduces the risk of stroke by 62 % , and that <span class="chemical">aspirin</span> alone reduces the risk by 22 % . 