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: Two patients with signs of carbamazepine neurotoxicity after combined treatment with verapamil showed complete recovery after discontinuation of the calcium entry blocker .
### Output Text:
Two patients with signs of <span class="chemical">carbamazepine</span> neurotoxicity after combined treatment with <span class="chemical">verapamil</span> showed complete recovery after discontinuation of the <span class="chemical">calcium</span> entry blocker . 