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: When tacrolimus side effects persist despite dose reduction , conversion to cyclosporine - based immunosuppression ( CyA ) is necessary .
### Output Text:
When <span class="chemical">tacrolimus</span> side effects persist despite dose reduction , conversion to <span class="chemical">cyclosporine</span> - based immunosuppression ( CyA ) is necessary . 