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: Dextromethorphan ( DM ) , the dextrorotatory isomer of 3 - hydroxy - N - methylmorphinan , is the main ingredient in a number of widely available , over - the - counter antitussives .
### Output Text:
<span class="chemical">Dextromethorphan</span> ( <span class="chemical">DM</span> ) , the dextrorotatory isomer of <span class="chemical">3 - hydroxy - N - methylmorphinan</span> , is the main ingredient in a number of widely available , over - the - counter antitussives . 