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: CONCLUSIONS : Caution is recommended in combining clonidine and verapamil therapy , even in patients who do not have sinus or AV node dysfunction .
### Output Text:
CONCLUSIONS : Caution is recommended in combining <span class="chemical">clonidine</span> and <span class="chemical">verapamil</span> therapy , even in patients who do not have sinus or AV node dysfunction . 