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: This 22 - year - old epileptic woman was treated continuously by carbamazepine and a folic acid ( 1 mg ) - containing multivitamin from the 20th week of gestation .
### Output Text:
This 22 - year - old epileptic woman was treated continuously by <span class="chemical">carbamazepine</span> and a <span class="chemical">folic acid</span> ( 1 mg ) - containing multivitamin from the 20th week of gestation . 