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 use of cocaine and / or amphetamine is a strong risk factor for stroke in this socioeconomically heterogeneous , insured urban population .
### Output Text:
The use of <span class="chemical">cocaine</span> and / or <span class="chemical">amphetamine</span> is a strong risk factor for stroke in this socioeconomically heterogeneous , insured urban population . 