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 age and sex of the control subjects did not differ from those of the methamphetamine dependence patients .
### Output Text:
The age and sex of the control subjects did not differ from those of the <span class="chemical">methamphetamine</span> dependence patients . 