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: The patients who had been exposed to appetite suppressants tended to be on average more severely ill , and to have a shorter median delay between onset of symptoms and diagnosis .
### Output Text:
The patients who had been exposed to <span class="chemical">appetite suppressants</span> tended to be on average more severely ill , and to have a shorter median delay between onset of symptoms and diagnosis . 