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: We report a favorable response to treatment with citalopram by a 15 - year - old boy with major depression who exhibited palpebral twitching during his first 2 weeks of treatment .
### Output Text:
We report a favorable response to treatment with <span class="chemical">citalopram</span> by a 15 - year - old boy with major depression who exhibited palpebral twitching during his first 2 weeks of treatment . 