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: Physicians and patients should be aware of this adverse effect so that , upon occurrence , they can discontinue methimazole therapy and avoid unnecessary invasive procedures .
### Output Text:
Physicians and patients should be aware of this adverse effect so that , upon occurrence , they can discontinue <span class="chemical">methimazole</span> therapy and avoid unnecessary invasive procedures . 