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: Seizure frequencies before and after initiation of levetiracetam treatment were compared , and adverse effects were recorded .
### Output Text:
Seizure frequencies before and after initiation of <span class="chemical">levetiracetam</span> treatment were compared , and adverse effects were recorded . 