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: In a 6 - week double - blind parallel treatment study , dothiepin and amitriptyline were compared to placebo in the treatment of 33 depressed outpatients .
### Output Text:
In a 6 - week double - blind parallel treatment study , <span class="chemical">dothiepin</span> and <span class="chemical">amitriptyline</span> were compared to placebo in the treatment of 33 depressed outpatients . 