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: Among women who used oral contraceptives , the odds ratio was 2 . 1 ( 95 percent confidence interval , 1 . 5 to 3 . 0 ) for those without a prothrombotic mutation and 1 . 9 ( 95 percent confidence interval , 0 . 6 to 5 . 5 ) for those with a mutation CONCLUSIONS : The risk of myocardial infarction was increased among women who used second - generation oral contraceptives .
### Output Text:
Among women who used <span class="chemical">oral contraceptives</span> , the odds ratio was 2 . 1 ( 95 percent confidence interval , 1 . 5 to 3 . 0 ) for those without a prothrombotic mutation and 1 . 9 ( 95 percent confidence interval , 0 . 6 to 5 . 5 ) for those with a mutation CONCLUSIONS : The risk of myocardial infarction was increased among women who used second - generation <span class="chemical">oral contraceptives</span> . 