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: RESULTS : All patients had optic neuropathy between 1 to 6 months ( mean = 2 . 9 months ) after starting ethambutol therapy at a dosage ranging from 13 to 20 mg / kg / day ( mean = 17 mg / kg / day ) .
### Output Text:
RESULTS : All patients had optic neuropathy between 1 to 6 months ( mean = 2 . 9 months ) after starting <span class="chemical">ethambutol</span> therapy at a dosage ranging from 13 to 20 mg / kg / day ( mean = 17 mg / kg / day ) . 