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: CONCLUSION : Those using methadone and heroin simultaneously may increase risk of rhabdomyolysis and ischemic stroke .
### Output Text:
CONCLUSION : Those using <span class="chemical">methadone</span> and <span class="chemical">heroin</span> simultaneously may increase risk of rhabdomyolysis and ischemic stroke . 