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: Hypotheses of heroin - related rhabdomyolysis and stroke in heroin abusers are discussed .
### Output Text:
Hypotheses of <span class="chemical">heroin</span> - related rhabdomyolysis and stroke in <span class="chemical">heroin</span> abusers are discussed . 