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: The blood tacrolimus concentration was higher than usual at that time ; thus , tacrolimus dosage was reduced .
### Output Text:
The blood <span class="chemical">tacrolimus</span> concentration was higher than usual at that time ; thus , <span class="chemical">tacrolimus</span> dosage was reduced . 