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: Use of verapamil in combination with carbamazepine should either be avoided or prescribed only with appropriate adjustment of the carbamazepine dose ( usually reduction of the carbamazepine dose by one half ) .
### Output Text:
Use of <span class="chemical">verapamil</span> in combination with <span class="chemical">carbamazepine</span> should either be avoided or prescribed only with appropriate adjustment of the <span class="chemical">carbamazepine</span> dose ( usually reduction of the <span class="chemical">carbamazepine</span> dose by one half ) . 