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: In the seven men who completed six months on thalidomide , subclinical evidence of peripheral neuropathy was found in four before treatment , but in all seven at repeat testing .
### Output Text:
In the seven men who completed six months on <span class="chemical">thalidomide</span> , subclinical evidence of peripheral neuropathy was found in four before treatment , but in all seven at repeat testing . 