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: Fifty - six days after initiation of the treatment the patient presented hallucinations that only stopped after the withdrawal of psycho - active drugs and tramadol .
### Output Text:
Fifty - six days after initiation of the treatment the patient presented hallucinations that only stopped after the withdrawal of psycho - active drugs and <span class="chemical">tramadol</span> . 