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: The epidemiological studies that assessed the risk of venous thromboembolism ( VTE ) associated with newer oral contraceptives ( OC ) did not distinguish between patterns of OC use , namely first - time users , repeaters and switchers .
### Output Text:
The epidemiological studies that assessed the risk of venous thromboembolism ( VTE ) associated with newer <span class="chemical">oral contraceptives</span> ( <span class="chemical">OC</span> ) did not distinguish between patterns of <span class="chemical">OC</span> use , namely first - time users , repeaters and switchers . 