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: It may even present in patients who have tolerated this medicine well in the past .
### Output Text:
It may even present in patients who have tolerated this medicine well in the past . 