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: Fusidic acid was administered orally in a dose of 500 mg t . d . s . and the treatment was planned to last 8 weeks .
### Output Text:
<span class="chemical">Fusidic acid</span> was administered orally in a dose of 500 mg t . d . s . and the treatment was planned to last 8 weeks . 