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: After discontinuing isoniazid therapy a similar pattern of behavior was noted that was controlled by pyridoxine .
### Output Text:
After discontinuing <span class="chemical">isoniazid</span> therapy a similar pattern of behavior was noted that was controlled by <span class="chemical">pyridoxine</span> . 