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: Our patient started to have symptoms of RLS only after he had been treated with mirtazapine , and his RLS symptoms resolved completely upon discontinuation of his mirtazapine .
### Output Text:
Our patient started to have symptoms of RLS only after he had been treated with <span class="chemical">mirtazapine</span> , and his RLS symptoms resolved completely upon discontinuation of his <span class="chemical">mirtazapine</span> . 