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 : We conducted a retrospective cohort study using administrative data of patients > or = 65 years of age who filled a prescription for NSAID or acetaminophen during 1999 - 2002 .
### Output Text:
METHODS : We conducted a retrospective cohort study using administrative data of patients > or = 65 years of age who filled a prescription for NSAID or <span class="chemical">acetaminophen</span> during 1999 - 2002 . 