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 : Mean ( SD ; maximum ) duration of treatment was 19 . 3 ( 10 . 3 ; 32 . 9 ) and 19 . 1 ( 10 . 4 ; 33 . 1 ) months in the etoricoxib and diclofenac groups , respectively .
### Output Text:
RESULTS : Mean ( SD ; maximum ) duration of treatment was 19 . 3 ( 10 . 3 ; 32 . 9 ) and 19 . 1 ( 10 . 4 ; 33 . 1 ) months in the <span class="chemical">etoricoxib</span> and <span class="chemical">diclofenac</span> groups , respectively . 