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: To characterize the nature , time course and dose dependency of zidovudine - related side effects , we undertook a multicenter , prospective , dose - range finding study .
### Output Text:
To characterize the nature , time course and dose dependency of <span class="chemical">zidovudine</span> - related side effects , we undertook a multicenter , prospective , dose - range finding study . 