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: The full syndrome of subacute myelo - optic neuropathy was more frequent in women , but they tended to have taken greater quantities of the drug .
### Output Text:
The full syndrome of subacute myelo - optic neuropathy was more frequent in women , but they tended to have taken greater quantities of the drug . 