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: Vitamin B12 and folinic acid supplementation of ZDV therapy does not seem useful in preventing or reducing ZDV - induced myelotoxicity in the overall treated population , although a beneficial effect in certain subgroups of patients cannot be excluded .
### Output Text:
<span class="chemical">Vitamin B12</span> and <span class="chemical">folinic acid</span> supplementation of <span class="chemical">ZDV</span> therapy does not seem useful in preventing or reducing <span class="chemical">ZDV</span> - induced myelotoxicity in the overall treated population , although a beneficial effect in certain subgroups of patients cannot be excluded . 