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 aim of this study was to assess the effects of gabapentin , a drug effective in neuropathic pain patients , on brain processing of nociceptive information in normal and central sensitization states .
### Output Text:
The aim of this study was to assess the effects of <span class="chemical">gabapentin</span> , a drug effective in neuropathic pain patients , on brain processing of nociceptive information in normal and central sensitization states . 