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: Almost 36 % of patients taking rizatriptan were pain free at 2 h and had no recurrence or need for additional medication within 24 h , compared to 20 % of patients on ergotamine / caffeine ( p < or = 0 . 001 ) .
### Output Text:
Almost 36 % of patients taking <span class="chemical">rizatriptan</span> were pain free at 2 h and had no recurrence or need for additional medication within 24 h , compared to 20 % of patients on <span class="chemical">ergotamine</span> / <span class="chemical">caffeine</span> ( p < or = 0 . 001 ) . 