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 case report questions the long term use of pain killers combined with psycho - active drugs in chronic non malignant pain , especially if pain is under control .
### Output Text:
The case report questions the long term use of pain killers combined with psycho - active drugs in chronic non malignant pain , especially if pain is under control . 