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 results show an association between bendrofluazide treatment and impotence , and impotence also occurred more frequently in patients taking propranolol than in those taking placebos .
### Output Text:
The results show an association between <span class="chemical">bendrofluazide</span> treatment and impotence , and impotence also occurred more frequently in patients taking <span class="chemical">propranolol</span> than in those taking placebos . 