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: Neuropathy may thus be a common complication of thalidomide in older patients .
### Output Text:
Neuropathy may thus be a common complication of <span class="chemical">thalidomide</span> in older patients . 