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: Administration of doses over 800 mg / day was difficult to achieve in this patient population , however lower doses were practical .
### Output Text:
Administration of doses over 800 mg / day was difficult to achieve in this patient population , however lower doses were practical . 