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: A metabolic drug interaction between flecainide and paroxetine , which the patient had been taking for more than 5 years , was considered .
### Output Text:
A metabolic drug interaction between <span class="chemical">flecainide</span> and <span class="chemical">paroxetine</span> , which the patient had been taking for more than 5 years , was considered . 