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: METHODS : Using the General Practice Research Database we conducted a cohort analysis and case - control study nested within a population of women aged between 15 and 39 years with acne , hirsutism or PCOS to estimate the risk of VTE associated with CPA / EE .
### Output Text:
METHODS : Using the General Practice Research Database we conducted a cohort analysis and case - control study nested within a population of women aged between 15 and 39 years with acne , hirsutism or PCOS to estimate the risk of VTE associated with <span class="chemical">CPA</span> / <span class="chemical">EE</span> . 