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: To avoid disastrous outcomes , physicians must consider heparin - induced thrombocytopenia whenever a recently hospitalized patient returns with thromboembolism ; therapy with alternative anticoagulants , not heparin , should be initiated .
### Output Text:
To avoid disastrous outcomes , physicians must consider <span class="chemical">heparin</span> - induced thrombocytopenia whenever a recently hospitalized patient returns with thromboembolism ; therapy with alternative anticoagulants , not <span class="chemical">heparin</span> , should be initiated . 