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: The fits ceased within 4 hours of administering intramuscular pyridoxine , suggesting an aetiology of pyridoxine deficiency secondary to isoniazid medication .
### Output Text:
The fits ceased within 4 hours of administering intramuscular <span class="chemical">pyridoxine</span> , suggesting an aetiology of <span class="chemical">pyridoxine</span> deficiency secondary to <span class="chemical">isoniazid</span> medication . 