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: A placebo had no effect , but niacinamide was as effective as pyridoxine .
### Output Text:
A placebo had no effect , but <span class="chemical">niacinamide</span> was as effective as <span class="chemical">pyridoxine</span> . 