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: METHODS : According to a double - blind , randomised , controlled , threefold cross - over design , 18 healthy subjects ( 11 males , 7 females ; mean age 26 years ) received either placebo , 400 mg ibuprofen , or 800 mg ibuprofen .
### Output Text:
METHODS : According to a double - blind , randomised , controlled , threefold cross - over design , 18 healthy subjects ( 11 males , 7 females ; mean age 26 years ) received either placebo , 400 mg <span class="chemical">ibuprofen</span> , or 800 mg <span class="chemical">ibuprofen</span> . 