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: Median seizure frequency prior to treatment with levetiracetam ( 2 . 1 seizures / mo ) was significantly higher than median seizure frequency after initiation of levetiracetam treatment ( 0 . 42 seizures / mo ) , and 7 of 10 cats were classified as having responded to levetiracetam treatment ( ie , reduction in seizure frequency of > or = 50 % ) .
### Output Text:
Median seizure frequency prior to treatment with <span class="chemical">levetiracetam</span> ( 2 . 1 seizures / mo ) was significantly higher than median seizure frequency after initiation of <span class="chemical">levetiracetam</span> treatment ( 0 . 42 seizures / mo ) , and 7 of 10 cats were classified as having responded to <span class="chemical">levetiracetam</span> treatment ( ie , reduction in seizure frequency of > or = 50 % ) . 