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: Toxic hepatitis induced by antithyroid drugs : four cases including one with cross - reactivity between carbimazole and benzylthiouracil .
### Output Text:
Toxic hepatitis induced by antithyroid drugs : four cases including one with cross - reactivity between <span class="chemical">carbimazole</span> and <span class="chemical">benzylthiouracil</span> . 