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: Important untoward effects of heparin therapy including heparin - induced thrombocytopenia , heparin - associated osteoporosis , eosinophilia , skin reactions , allergic reactions other than thrombocytopenia and alopecia will be discussed in this article .
### Output Text:
Important untoward effects of <span class="chemical">heparin</span> therapy including <span class="chemical">heparin</span> - induced thrombocytopenia , <span class="chemical">heparin</span> - associated osteoporosis , eosinophilia , skin reactions , allergic reactions other than thrombocytopenia and alopecia will be discussed in this article . 