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: These findings indicate that gabapentin has a measurable antinociceptive effect and a stronger antihyperalgesic effect most evident in the brain areas undergoing deactivation , thus supporting the concept that gabapentin is more effective in modulating nociceptive transmission when central sensitization is present .
### Output Text:
These findings indicate that <span class="chemical">gabapentin</span> has a measurable antinociceptive effect and a stronger antihyperalgesic effect most evident in the brain areas undergoing deactivation , thus supporting the concept that <span class="chemical">gabapentin</span> is more effective in modulating nociceptive transmission when central sensitization is present . 