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: Two groups of patients receiving tacrolimus were compared over a period of 1 year , one group comprising hypertensive patients who were receiving nifedipine , and the other comprising nonhypertensive patients not receiving nifedipine .
### Output Text:
Two groups of patients receiving <span class="chemical">tacrolimus</span> were compared over a period of 1 year , one group comprising hypertensive patients who were receiving <span class="chemical">nifedipine</span> , and the other comprising nonhypertensive patients not receiving <span class="chemical">nifedipine</span> . 