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 : As described earlier , administration of ibuprofen was followed by a decrease in tonic pain but - relative to placebo - an increase in correlates of phasic pain , indicating a specific effect of ibuprofen on the interaction between the pain stimuli under these special experimental conditions .
### Output Text:
RESULTS : As described earlier , administration of <span class="chemical">ibuprofen</span> was followed by a decrease in tonic pain but - relative to placebo - an increase in correlates of phasic pain , indicating a specific effect of <span class="chemical">ibuprofen</span> on the interaction between the pain stimuli under these special experimental conditions . 