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 effect of nifedipine on renal function in liver transplant recipients who were receiving tacrolimus was evaluated between January 1992 and January 1996 .
### Output Text:
The effect of <span class="chemical">nifedipine</span> on renal function in liver transplant recipients who were receiving <span class="chemical">tacrolimus</span> was evaluated between January 1992 and January 1996 . 