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: OBJECTIVE : To study the effect of folic acid - containing multivitamin supplementation in epileptic women before and during pregnancy in order to determine the rate of structural birth defects and epilepsy - related side effects .
### Output Text:
OBJECTIVE : To study the effect of <span class="chemical">folic acid</span> - containing multivitamin supplementation in epileptic women before and during pregnancy in order to determine the rate of structural birth defects and epilepsy - related side effects . 