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: OBJECTIVE : To examine the safety and tolerability of clonidine used alone or with methylphenidate in children with attention - deficit / hyperactivity disorder ( ADHD ) .
### Output Text:
OBJECTIVE : To examine the safety and tolerability of <span class="chemical">clonidine</span> used alone or with <span class="chemical">methylphenidate</span> in children with attention - deficit / hyperactivity disorder ( ADHD ) . 