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: Rifampicin was administered thrice as one of the 3 - 4 drug regimen and each time he developed untoward side effects like nausea , vomiting and fever with chills and rigors .
### Output Text:
<span class="chemical">Rifampicin</span> was administered thrice as one of the 3 - 4 drug regimen and each time he developed untoward side effects like nausea , vomiting and fever with chills and rigors . 