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: In this paper we aim to draw to the fact that caution is needed when prescribing vitamin A - containing drugs to women of childbearing years .
### Output Text:
In this paper we aim to draw to the fact that caution is needed when prescribing <span class="chemical">vitamin A</span> - containing drugs to women of childbearing years . 