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: Doses were flexibly titrated up to 0 . 6 mg / day for clonidine and 60 mg / day for methylphenidate ( both with divided dosing ) .
### Output Text:
Doses were flexibly titrated up to 0 . 6 mg / day for <span class="chemical">clonidine</span> and 60 mg / day for <span class="chemical">methylphenidate</span> ( both with divided dosing ) . 