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: A patient who received tocolysis with nifedipine developed neuromuscular blockade after 500 mg of magnesium sulfate was administered .
### Output Text:
A patient who received tocolysis with <span class="chemical">nifedipine</span> developed neuromuscular blockade after 500 mg of <span class="chemical">magnesium sulfate</span> was administered . 