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: Significant obsessive - compulsive behavior ensued but diminished over several weeks when methylphenidate was replaced by fluvoxamine .
### Output Text:
Significant obsessive - compulsive behavior ensued but diminished over several weeks when <span class="chemical">methylphenidate</span> was replaced by <span class="chemical">fluvoxamine</span> . 