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: Cardiovascular risk with cyclooxygenase inhibitors : general problem with substance specific differences ?
### Output Text:
Cardiovascular risk with <span class="chemical">cyclooxygenase inhibitors</span> : general problem with substance specific differences ? 