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: The dose of gabapentin was 1 , 800 mg per os , in a single administration .
### Output Text:
The dose of <span class="chemical">gabapentin</span> was 1 , 800 mg per os , in a single administration . 