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: Before dobutamine was begun , no therapy was given during the first visit , and patients were randomized on subsequent visits to receive metoprolol or metoprolol plus glucagon 1 mg .
### Output Text:
Before <span class="chemical">dobutamine</span> was begun , no therapy was given during the first visit , and patients were randomized on subsequent visits to receive <span class="chemical">metoprolol</span> or <span class="chemical">metoprolol</span> plus glucagon 1 mg . 