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: Although large - dose amiodarone is highly effective in the long - term treatment of VT or VF refractory to conventional antiarrhythmic drugs , it causes significant toxicity in approximately 50 % of patients .
### Output Text:
Although large - dose <span class="chemical">amiodarone</span> is highly effective in the long - term treatment of VT or VF refractory to conventional antiarrhythmic drugs , it causes significant toxicity in approximately 50 % of patients . 