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: Cardiomyopathy is frequent when the total dose exceeds 600 mg / m2 and occurs within one to six months after cessation of therapy .
### Output Text:
Cardiomyopathy is frequent when the total dose exceeds 600 mg / m2 and occurs within one to six months after cessation of therapy . 