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: Randomised clinical trials and observational studies have shown an increased risk of myocardial infarction , stroke , hypertension and heart failure during treatment with cyclooxygenase inhibitors .
### Output Text:
Randomised clinical trials and observational studies have shown an increased risk of myocardial infarction , stroke , hypertension and heart failure during treatment with <span class="chemical">cyclooxygenase inhibitors</span> . 