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: CONCLUSIONS : Clonidine , used alone or with methylphenidate , appears safe and well tolerated in childhood ADHD .
### Output Text:
CONCLUSIONS : <span class="chemical">Clonidine</span> , used alone or with <span class="chemical">methylphenidate</span> , appears safe and well tolerated in childhood ADHD . 