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: But before this event , she was taking zolmitriptan 2 - 3 times daily for several days because of a persistent migraine headache .
### Output Text:
But before this event , she was taking <span class="chemical">zolmitriptan</span> 2 - 3 times daily for several days because of a persistent migraine headache . 