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: Both systolic and diastolic blood pressures of these 13 patients were decreased significantly after 4 weeks of nifedipine therapy , and blood pressure was maintained within the normal range thereafter for 25 months .
### Output Text:
Both systolic and diastolic blood pressures of these 13 patients were decreased significantly after 4 weeks of <span class="chemical">nifedipine</span> therapy , and blood pressure was maintained within the normal range thereafter for 25 months . 