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: Although mazindol - treated patients gained less weight and height than placebo - treated patients , no significant effect on IGF - I levels was observed .
### Output Text:
Although <span class="chemical">mazindol</span> - treated patients gained less weight and height than placebo - treated patients , no significant effect on IGF - I levels was observed . 