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: After cessation of the sulfasalazine and completion of a six - week course of corticosteroids , these problems resolved over a period of four to six months .
### Output Text:
After cessation of the <span class="chemical">sulfasalazine</span> and completion of a six - week course of corticosteroids , these problems resolved over a period of four to six months . 