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: Valproic acid ( VPA ) is effective for the treatment of many types of epilepsy , but its use can be associated with an increase in body weight .
### Output Text:
<span class="chemical">Valproic acid</span> ( <span class="chemical">VPA</span> ) is effective for the treatment of many types of epilepsy , but its use can be associated with an increase in body weight . 